In May, 1979, on a street in SoHo, Etan Patz, 6, who had begged his mother to walk to the school bus alone for the first time, disappeared.

He vanished, police said later, "without a trace," and for months -- years -- he was the best known missing child in the city. Posters of the blond, blue-eyed boy seemed to be on nearly every block downtown. Ads appeared in the foreign-language newspapers. Four detectives, after an initial force of 500, were assigned to the case permanently.

Anniversaries of Patz' disappearance were marked by interviews with his parents, who had kept an open telephone line to the public and every year said the same thing: they knew their son was alive. They were keeping the top of his bunk bed for him, waiting. They had not given up hope.

During the holiday season, when the city was primed for a happy ending, the Etan Patz mystery came alive again.

Police in Massachusetts, in a raid on a homosexual group called the North American Man-Boy Love Association, found a photo of a boy with what they called a "remarkable resemblance" to Etan. Last week, 3 1/2 years after the child disappeared, a retired cab driver walked into The New York Daily News and announced that he believed he had picked up Etan Patz and his abductor, a man he said was between 20 and 30.

Whether any of this will lead to Etan Patz is uncertain, particuarly since Etan's parents have said the boy in the photos is not their son.

Nonetheless, it has all reopened a wound, with New Yorkers, despite themselves, hoping.

Etan Patz was, or is, a middle-class city child. His father, Stan, is a photographer, his mother, Julie, ran a day-care center in their SoHo loft. When the search for Etan began, the largest manhunt for a child in recent history, there were those who claimed that it was all because Etan was a middle-class white boy.

That he was a middle-class white boy undoubtedly did not hurt.

Nor did the publicity generated by the efforts of his family's well-educated, well-connected SoHo community.

But perhaps what finally made the case so powerful here was that the disappearance of Etan was the urban parent's nightmare: a child, walking down a crowded city street, is abducted by a stranger whom no one can quite remember. A child, walking along a familiar route, only one block long, never makes it to his destination.

The circumstances of the crime also struck a chord. Etan was not the child of uncaring parents.

But on the morning of his disappearance, according to a story in The New York Times, his mother had her hands full. Etan's older sister was lingering at home, his 2-year-old brother had a sleep-over friend, his father was asleep. Etan had asked to walk to school alone many times before. On May 25, the day of his disappearance, his mother finally gave her approval.

After word of the Massachusetts sex ring appeared, a cab driver, Chester Jones, who police say is a credible witness, came forward. He said he had never done so before because he didn't want to get involved.

He said he felt guilty about having remained silent and was troubled by his conscience. He also, according to The Daily News, recounted a story about the abduction that fleshed out the urban nightmare, making a parent's worst fantasies real.

Jones said he heard the man tell the boy he "saw him every morning across the street" and that it was "a shame your mother lets you stand here on the street corner all alone."

He heard the boy, in "a very, very soft voice," telling the man that his mother had told him never to talk to strangers and that "this isn't the way to school." When the boy told the man that, Jones added, the man told Jones to stop the cab, and they got out.

But this development has not proven conclusive, according to Lt. Earl Campazzi of the Missing Persons Bureau.

The cabbie was not able to produce enough information for police to provide a composite sketch, he said wearily, nor was the man able to select a suspect from police photos.

"It fills in a gap," said Campazzi. "It doesn't lead to an ultimate conclusion."

A far stronger lead, the officer says, may prove to be the photo from the sex ring -- another part of the urban nightmare.

Today, David Thorstad, a founder of the sex ring, said the photograph was first published in a 1968 calendar and could not be that of Etan Patz. Although the Patz family has said the boy in the photo is not Etan, New York police are forwarding the picture to the FBI and are awaiting positive identification.

Just to be sure they have gotten every possible bit of information from the cabbie, the police said they will have him hypnotized.

"If he'd come to us earlier, it would have helped immensely . . . . It would have given us a definite direction in the route of the child, firmed up our eyewitnesses," says Campazzi, so tired that there are long pauses when he speaks. "Now . . . now. . . we just wait . . . . "