He is quiet-spoken and of medium build, a shy young man with a tenor voice and a lilt to his English pronunciation that marks him, like so many in this land of immigrants, as coming from someplace else in the world.

He goes to Steinmetz High School and works part time at a neighborhood Jewel supermarket. He is a genuine American teen-ager -- except that he is a Soviet defector.

And as of today, on his 18th birthday, Walter Polovchak will never have to worry about being forced to return to the homeland he has come to detest since he left it five years ago.

"There is no way I'd ever go back to that country. I got nothin' to go back for," says Polovchak. "My life is here in America. There is nothing -- nothing -- I miss in the Soviet Union."

His coming of age alone will end for good the protracted legal struggle that has dogged the teen-ager's life ever since he stunned his family -- and caused world headlines -- in 1980. It was his decision then that he did not want to return to the U.S.S.R. with his parents and younger brother. They had emigrated here from the Ukraine, then changed their minds and headed back to the Soviet Union.

As an 18-year-old, Walter will be freed under Soviet law of his parents' legal control and recognized as well in American law to be an adult with legal power to decide for himself where he wishes to live. He has decided to seek U.S. citizenship.

In so many ways that mock the usual parent-child wars of American adolescence, Polovchak has fought steadfastly, unswervingly, for total freedom from his parents. For much of the time, it has been a struggle at a 5,000-mile range, as his parents, now again living in the provincial Ukrainian city of Sambir, sought to assert their parental rights to have their son live with them.

Through all the legal battles, Walter has lived with relatives and his older sister, Natalie, 22, here in the heart of Chicago's Ukrainian community on the Northwest side, pursuing as quiet an existence as most other American adolescents.

There is no rancor in his voice when he talks about his new homeland -- the United States. "I will register for the draft," he says. "I'm an American. It's just like I was born here. Part of the responsibility of an American is to register for the draft. And if I ever got drafted, I'd be more than glad to serve."

When he arrived in the country five years ago, Walter was 12, still a small boy, wearing glasses, pudgy-faced, a season or two shy of the adolescent growth spurt that would make angles of the round curves of face and frame.

He hardly looked the stuff of international headlines, a child whose strength of purpose would contribute to rising tensions between the mightiest adversaries the world has ever known.

He had come to America with his parents, Michael and Anna Polovchak; Natalie; and his little brother, Michael. His parents had talked of emigrating to the United States as a number of relatives had done since the end of World War II. His father, a bus driver, was seeking a better life -- like millions of immigrants before him.

The simple fact that the Polovchaks obtained permission to leave the Motherland was feat enough: The U.S.S.R.'s borders are closed and guarded and few citizens are ever allowed to depart simply because they wish to live elsewhere.

But in the 1970s, about 350,000 Soviets, mostly Jews, Armenians and ethnic Germans, were allowed to leave. The Polovchaks, ethnic Ukrainians, were part of this stream. Like thousands of Soviet e'migre's, they chose the United States over any other place to live.

But almost from the day the family arrived, there were strains. Life in the capitalist West turned out to be more demanding than the Polovchaks had imagined. This in itself was nothing unusual for immigrant families. What was unusual is that Michael Polovchak began talking about returning. Very few Soviet e'migre's take so drastic a step. But Michael Polovchak began phoning the Soviet Embassy's consulate section in Washington, making plans to return to his native land.

Even more unusual, Walter began making his own plans -- to leave his parents and live with relatives here rather than step back over his native land's closed borders for good.

"It's real hard to explain to Americans what it's like inside," Walter says of life in his homeland. "You've gotta experience it to understand it.

"Here's the idea: In this country, my America, I could go to any city I wanted without being bothered by the authorities. I could go to church. I don't need an internal passport. It's real hard to explain to Americans what it's like in a place where you don't have those rights."

When the family headed for home, Walter hid out, sheltering with relatives and friends willing to defend his right to stay behind. The State Department intervened on his behalf, granting him asylum. The Kremlin denounced it as kidnaping. His parents pleaded with him to obey.

He refused. When his parents left, he stayed behind. "I got nothing to go back for," he says.

There is a memory Walter Polovchak carries in his recollections of life in Sambir, in the western Ukraine. He has retold this scene many times: Walter and his mother go to church in Sambir. They are Ukrainian Catholics. Christians are known as "believers" in the U.S.S.R.

When he arrives at church, Walter sees some familiar faces at the very front of the congregation, coldly surveying the crowd -- teachers from the local schools. Trouble ahead.

"There are only 10 or 11 schools in the whole city. The teachers know everyone. They see you, they write your name down, and then you get punished. Recite the rules of Lenin. Scrub the floors. Clean the graffiti. There are all kinds of punishments."

Penance for going to church.

"Here, you can go into any church, believe in any religion. It's definitely better here."

The boy's lawyer, Julian Kulas, an activist in Chicago's large Ukrainian community, told judges at various hearings the child likely would be punished severely by Soviet authorities if he returned.

But Walter's refusal to accompany his parents stirred an unusual legal fight that many people found distasteful. The Chicago branch of the American Civil Liberties Union went to court on behalf of his parents, attacking the U.S. government for granting him asylum.

The government had acted in a way that fostered family breakup, the ACLU said.

"If they send me back, I'll commit suicide," Walter said at the time.

The dispute brought the child known here as "The Littlest Defector" into court -- and hence, into the news -- numerous times.

"When you're news, you're news," Walter reflects. "Then, you're just a normal person. Then, you're news again. It gets to you after awhile. You feel like you're a tape recorder, saying the same things over, and over, and over."

But his real interest was to be a normal American kid. He was put back a grade in public school because of the language barrier. He hacked around with some other American kids, got into a few scrapes with the authorities for pranks, then began to buckle down at his studies and his future.

His sister works now as a cook in the employe's cafeteria of a suburban Neiman-Marcus store. Walter is in his senior year at Steinmetz and works about 20 hours a week as a carryout clerk at the supermarket.

The two have taken up residence with a cousin, Walter Polowczak, who spells his last name the Polish way. The cousin, a 29-year-old computer software engineer, notes that in the part of Russia where the Polovchak relatives were located, "the border moved around a lot."

In fact, the winds of war have played their own havoc in scattering or destroying most of the child's extended family. Several uncles were killed by the Germans during Hitler's invasion of Russia. An aunt was taken to Germany by the Nazis and forced to work in a labor camp there. She survived and came to the United States after the war.

Cousin Walter Polowczak credits a grandfather who served in the Austro-Hungarian Empire's army during World War I for instilling in the family the notion of living where one pleases.

"This grandfather was a rather intelligent person. He was well-cultured and very honest. He'd seen a bit of the world. I knew one thing from childhood: I didn't want to live in the Soviet Union." Cousin Polowczak and his mother emigrated in 1968.

With his birthday, Walter need never worry about emigration again. "I can't complain," he says. "I just want to get a high school degree, and figure out what college I want to go to.

"Maybe I'll study computers, or computer repair . . . something like that."

He says he has not heard from his parents in the past 18 months. "They don't write to me or nothing." But his little brother, Michael, 10, wrote recently and sent pictures.

Says Walter, "My mother has aged quite a bit."