Ten years ago, Sheila and Katherine Lyon, two blond-haired sisters from Kensington, disappeared.

The girls' older brother, Jay, was the last to see his two sisters, then aged 12 and 10. They were sharing a pizza at the Orange Bowl in Wheaton Plaza about 2 p.m. on March 25, 1975, Jay Lyon said. And then they were gone.

"Essentially, they disappeared from the face of the earth," said Montgomery County Police Sgt. Steven Hargrove. "There was no discord at home, no word of leaving, they just left one afternoon and never returned."

While the chance that Sheila and Kate will be found safe grows slimmer each day, the issue of missing children has drawn increasing national attention. Today, for example, has been proclaimed National Missing Children's Day by President Reagan. Police and parents say they now have a better chance than ever of finding the estimated 1.5 million Americans under 18 who are considered missing.

Jay Howell, executive director of the National Center for Missing and Exploited Children in Washington, cites as proof the 870 children his federally funded agency has helped return home during its first year of operation.

"The difference is night and day," said Howell. "We are having much more success now.

"As recently as two years ago there weren't the technological resources, the private sector programs for displaying missing children's photos, or the federally funded hot line," he said.

But William C. Campbell, the Montgomery County police officer assigned to the Lyon case, says "there's only so much technology can do." And in the 1 to 2 percent of cases in which missing children are believed to have been abducted by strangers, the latest advances are of little help.

"It's important to let people know the scope of the problem," says John Lyon, the girls' father and a well-known announcer for WMAL radio. "I really don't know if all this will help in cases like ours."

The steps that law officers said are helping in other cases include:

* Broadening use of the FBI's National Crime Information computer, which makes local abduction reports instantly available to law enforcement officers across the country.

* Spending $4 million in federal funds to increase public awareness and law enforcement training.

* Increasing use of private sector funds and resources, such as milk cartons, shopping bags, bus bumpers -- even the flashing electronic billboard in New York's Times Square -- to display photographs and information about missing children.

This week, three separate congressional hearings were held to discuss the problem, and the Senate unanimously passed a bill that would print photographs and information about missing children on 75 percent of congressional mail.

But, parents and authorities agree, much is yet to be done.

"The field is still flooded with mini-information, lots of misinformation" and hucksters trying to cash in on the fears of parents, Howell said.

Along with clearing up those problems, Howell said the questions that need to be addressed are: "Who are these children and how can they be helped?"

According to the congressional definition adopted last October, a missing child is a person under 18 whose whereabouts are unknown to his legal custodians and whose circumstances indicate he or she may have been abducted, abused or sexually exploited.

Kenneth Wooden, the founder of the National Coalition for Children's Justice, proposed this week at a House Education and Labor subcommittee hearing that Congress categorize missing children in four main groups: those abducted by a stranger, those abducted by a parent, throwaways and runaways.

* Stranger abduction: By far the most rare and dangerous cases. Police estimate that only one percent of all missing children are abducted by a completely unfamiliar person. Studies show that there is little chance of survival if a child abducted by a stranger is not recovered within 48 hours.

Authorities in the field warn that teaching a child "to stay away from strangers" may be inadequate advice because those who kidnap children often do not appear "strange." Rather, they call the child by their first name and appear friendly. Experts said it is better to teach children to be suspicious of adults who offer their assistance or ask them if they want a ride.

* Parental abduction: The vast number of cases in which a child is kidnaped by a parent seems to mirror the soaring divorce rate. A 1984 survey conducted by the University of Rhode Island and pollster Louis Harris & Associates estimated that between 459,000 and 751,000 children are kidnaped by a parent who does not have legal custody of the child.

That's what happened to 3-year-old Justin Anthony of Alexandria.

Justin's father, Barry Anthony, failed to return Justin to his mother on July 1, 1984, after an agreed upon visit. First the father and son moved to Baltimore and then to Los Angeles, where police arrested Anthony on March 22.

"It's a myth that these kids don't need all our help," said James Chandler, director of the nonprofit Missing Children of Greater Washington. Even if they are not in physical danger, children kidnaped by the noncustodial parent are subjected to tremendous psychological problems, Chandler and others said.

Often they are told that the other spouse does not love them anymore, live a life on the run and are taught to distrust authority. They are frequently yanked away from school and friends.

"He wasn't hurt," said Brenda Anthony this week during an interview in her Alexandria home. "But he still tells me what the police did when they arrested his father," she said as Justin tugged at her skirts. "He's a smart little boy and he picks up everything he's told. For weeks after I got him back he would say things to me like 'You don't like me.' I know what kind of a person my husband is and what kind of influence he would have on Justin; that's why we separated. I'm just very lucky the police helped me."

Unlike the District of Columbia police, who do not arrest noncustodial parental abductors because the city's kidnaping statute specifically excludes parents, police in Maryland and Virginia consider such abductions misdemeanors unless the parent takes the child across state lines -- an automatic felony.

The District, which is rapidly gaining a national reputation as a haven for parental abductors, has angered many child advocates and legal professionals, including Howard Davidson, the director of the National Legal Resource Center for Child Advocacy and Protection, who says that differing state laws further complicate the search for missing children.

* Throwaways: Impossible to quantify because parents -- who usually report missing children -- are the culprits in these cases. The children are abandoned for a variety of reasons. Many are believed to be from low-income families. The problem of throwaway children is a relatively new area of study.

* Runaways: As many as 95 percent of missing children are believed, at least at first, to have left home voluntarily. They are notoriously difficult to count. The inspector general of the Department of Health and Human Services estimated in 1983 that there were 1,155,384 runaways or homeless youths in the United States.

Because they are not abducted or physically forced to leave home, a misconception persists that these children are not in immediate danger, said Howell.

"They are often the victims of street crime or sexual exploitation," Howell testified on the Hill this week. "Or they end up as homicide victims."

This vulnerable group is not playing out "some Huckleberry Finn fantasy," said William Bestpitch, a planner with the Virginia Division for Children. "They are at risk, and if they felt they had a safe, protective home to return to, they would."

Dr. Joseph J. Palombi, a child psychiatrist and a board member with Missing Children of Greater Washington, attributes the massive numbers of runaways to the high divorce rate and the same societal problems that are leading to increased depression and suicide among the nation's youth.

"Runaways are predominantly from the middle and upper class," he said. "They are sending up a red flag that something is wrong. It could be depression, strain, peer pressure, problems with school . . . They are saying 'help.' "