From the moment their son Scott was born, Ray and Karen Roberson have had to think often about their own deaths.

Scott entered the world 11 years ago with Down's syndrome and a faulty heart. Surgery repaired the heart, but Down's left him with an IQ in the mid- to high 50s. He is expected to be able to hold a menial job but not to manage the money he earns from it.

The Robersons, who live in Fairfax County, ask themselves the same bleak question that afflicts every parent, but particularly the mothers and fathers of a handicapped child: How will he care for himself after they die?

"It is something we often think about," said Karen Roberson, 40, a self-employed real estate agent. "Where is Scott going to live? What is going to happen to him?"

At the National Alliance for the Mentally Ill, that anxiety about "when I am gone" is so widespread that staff members have a shortcut way of referring to it. "When we say 'WIAG' around the office, everybody knows what it means," said lobbyist Richard T. Greer of Arlington, who has a mentally ill son.

A range of social trends and medical advances -- from the closing of mental wards to the development of life-prolonging surgery -- have made those "when I am gone" questions more urgent in recent years. So in this era of shrunken governmental assistance, some parents are looking for private organizations that will function as surrogate parents after the real ones are gone.

Twenty programs have sprung up across the country to provide guardianship and help in obtaining services for parents of handicapped children, according to Tony Apolloni, director of the California Institute on Human Services at Sonoma State University. Most are less than a decade old, but one group in Washington state has been operating for more than 20 years.

Six more help parents set up trust funds designed to care for their children without losing their government benefits, Apolloni said. In some cases, the programs also help the elderly plan for when they can no longer care for themselves.

The programs offer promise for parents of the mentally ill, the retarded or children with other developmental disabilities such as cerebral palsy, Apolloni said. But he warned that some of the newer programs have not proven that they have the endowment capital, stability and quality control they need. "What's to say they're really going to be there long term?" he asked.

Supporters reply that parents founded many of the groups and have a commitment to keep them going. "We're signing on for the long run," said Rhonda Buckner, coordinator of the Northern Virginia office of the nonprofit Planned Lifetime Assistance Network and its only paid staff member. The group is trying to raise a $3 million endowment to guarantee survival free of dependence on government money and to help low-income families who otherwise could not afford its services.

The Robersons signed up last year with PLAN, which was established three years ago by parents in Charlottesville. Ray Roberson's brother will be Scott's guardian if anything happens to the boy's parents, but Karen and Ray Roberson wanted backup help.

In December, Karen Roberson said, PLAN helped them draw up a list of the "basic everyday type things" they do for Scott -- driving him to the doctor, taking him to the beach and summer camp, attending school meetings. For a fee from a trust fund the Robersons are setting up, PLAN will arrange for someone to help Scott's guardian with those jobs and steer Scott through the government program bureaucracy. PLAN will not provide services directly because that would conflict with its mission to be an advocate, Buckner said.

Also, the Robersons intend to set up, with PLAN's help, a second fund -- a "family trust fund" -- for Scott, to provide extras, such as two weeks at a summer camp for handicapped children, that his government benefits would not cover. Their idea behind a trust fund is to avoid jeopardizing Scott's government benefits or bankrupting his sister, Betsy, 14, by giving the money directly to Scott in the parents' will.

Karen Roberson said she now has more peace of mind about Scott. "There will be someone for our relatives and Betsy to turn to," she said.

Greer also has signed up with PLAN and has already begun paying the program to look after his 22-year-old son. Greer, 59, said he cannot cope with his son's constant need for money, and Buckner's "professional objectivity" is better for both of them. "It's been an enormous relief to me," he said.

Edward J. Brazill of McLean, a National Aeronautics and Space Administration official, is chairman of the PLAN board and is signing up his 32-year-old schizophrenic son. "Right now, I see to it he has a place to live, that the rent is paid, that he spends his money on the right things," said Brazill, 66. "What I want is somebody to take over and do the kinds of things I am doing."

In Maryland, a similar organization, the Maryland Trust for Retarded Citizens, has signed up 217 clients, whose parents or guardians pay a $1,200 fee for an assurance of basic services after their death or disability. The trust, according to Chairman Charles Mobley, is developing programs to set up trust funds for a more complete package of basic services, and for extras.

In the District, the Association for Retarded Citizens is preparing an information packet for interested families but is not planning anything more extensive, a spokesman said.

The demand is heavy for what is sometimes called "life services planning." "We get 25 letters a month from all over the country wanting to know if we provide services in their area," said Paul L. Medlin, executive director of PACT Inc. in Chicago.

One reason for that, experts say, is the aging of the mental retardation advocacy movement. The young parents who founded the Association for Retarded Citizens in 1950 -- pushing first for school programs for young children, later for group homes for adults -- now face old age. "It's beginning to hit home to them that this is another need that they have," said Charlene Jones of the association's District chapter.

The closing of many wards of the big state hospitals in the last 25 years means that many mentally ill and retarded persons who once would have lived their lives on back wards with round-the-clock care are now in group homes or their parents' homes -- or homeless on the streets.

Medical advances such as the heart surgery Scott Roberson underwent will prolong the lives of many retarded and other handicapped people, who once died young. Brothers and sisters who once stayed in town now may move thousands of miles away, leaving no nearby relative to act as guardian, said Apolloni.

Laws and customs have changed too, although advocates say they have not changed enough. The adage was that it was best to disinherit handicapped children, Medlin said, and estate planning was seen as "a waste of time because their assets would go very quickly." More lawyers now know about setting up trust funds for the handicapped, although "there are still 10 who are not aware for every one who is," said New York lawyer Hyman Clurfeld, a specialist in the area.

Clurfeld and other lawyers warn, however, that laws in some states may make it impossible to create such trust funds, and that changes in laws could jeopardize their existence even in states that permit them.

Recent changes in guardianship laws in some states have helped parents, Apolloni said. Some states allow "limited guardianship," giving a guardian financial control but not taking away the handicapped person's civil rights or right to consent to or refuse medical treatment. Some now allow nonprofit groups, not just individuals or the state government, to become guardians.

Advocacy groups say Maryland, Virginia and the District still have antiquated guardianship laws, and they are lobbying for changes. PLAN hopes to assume guardianship for the disabled if the laws are changed.