Whenever a mini-breakthrough is announced in treating a widely publicized hereditary disease of children, it poses a difficult dilemma for doctors, parents and patients alike.

The disease in this case is Duchenne muscular dystrophy (DMD), the most common and most severe of the genetically transmitted disorders whose young victims comedian Jerry Lewis has championed in countless national telethons.

Last week, a promising, but limited, advance in the treatment of DMD was announced at a press conference at Johns Hopkins Hospital in Baltimore. Dr. Daniel B. Drachman, a neurologist who conducted the study, brought along two patients and their mother to discuss the everyday aspects of living with a progressive and ultimately fatal childhood disease that, at the state of today's medical knowledge, can neither be prevented nor cured. (It is possible, however, to determine by prenatal testing early in the mother's pregnancy whether a fetus is affected.)

The scientific message Drachman delivered was simple: Carefully monitored use of the common steroid drug prednisone has been found to defer, by as much as three years in some cases, the inevitable day when the DMD sufferer can no longer move about independently and becomes wheelchair-bound. In the typical untreated patient, this occurs at age 9 or 10; with prednisone, the day can be deferred until 12 or 13.

This part of Drachman's message was registered loud and clear on television programs that evening and in newspaper reports the following morning. What may not have sunk into the minds of listeners and readers was the physician-researcher's solemn warning about casual use of the prednisone treatment.

"Prednisone is a double-edged sword," Drachman said. "It has plenty of side effects and therefore you can't recommend it for all children with Duchenne dystrophy . . . Those, for example, who gain too much weight have to be excluded and {so do} those who become hyperactive . . ."

To parents and to pediatricians, Drachman offered this counsel: Wait till confirmatory studies -- now under way at four major medical centers -- are completed in about a year; by then it will be apparent just how effective the prednisone treatment really is. And maybe by that time there will be another drug treatment that gives equivalent results without equivalent side effects.

But for many patients and their families, a year seems too long to wait.

Duchenne muscular dystrophy is a sex-linked recessive genetic disease which, like the bleeding disease hemophilia, is carried on the X (female) chromosome. Females can be carriers, and, when they are pregnant with a son, have a 50 percent chance of transmitting the disorder to the child. Affected boys are born with the disease, but do not ordinarily show symptoms until they are 4 or 5. The onset of DMD is marked by noticeable weakening of the big upper arm and leg muscles. At an age when schoolmates are playing pickup ball in the schoolyard or trying out for Little League, the DMD child is tottering around unsteadily, by now aware that it will only be a matter of time before he can stand no longer. By the time other kids are getting ready for Boy Scout hikes and overnight campouts, he is firmly wheelchair bound.

As he enters adolescence, his muscles continue to weaken. If he is lucky, he has a motorized wheelchair to give him mobility -- motorized because his arms no longer have the strength to propel an unpowered chair. Most patients die in their mid-twenties.

Against this background, a treatment that promises to defer at least some of these milestones for two or three years represents a major advance. What parent would not leap at the chance to secure this reprieve for his or her son? And what pediatrician, moved by a young patient growing ever weaker, would not seriously consider offering this therapy to parent and child?

But as Drachman said, "No -- not yet; wait a year." And the Muscular Dystrophy Association has withheld its seal of approval for the prednisone treatment for the same reason: Carefully designed studies on a large scale have not yet been completed.

Dr. Sidney M. Wolfe, director of the Public Citizen Health Research Group, agreed in an interview that parents and doctors alike should restrain their enthusiasm for what seems a promising discovery until its potential is more firmly demonstrated.

Every time a new drug comes on the market, or a new use is recommended for an old one (like prednisone), doctors should wait a few years to see what unexpected side effects show up, says Wolfe.

Such advice is not always easily followed. Five years ago, a drug called Oraflex came on the market, touted as the answer to every arthritic's prayer. It was trumpeted across the land and within days physicians were flooded with patient demands for the new miracle drug. Only a few months later, Oraflex had to be taken off the market after reports that it caused damage to patients' livers and kidneys.

It is not hard to imagine a run on pediatricians' offices by parents of many of the 7,500 to 10,000 American children currently suffering from DMD, or of additional thousands of youngsters with other forms of muscular dystrophy for which prednisone's benefits are unknown. And it is not hard to imagine some doctors caving in to these parents' demands for what may sound like -- but is not -- a miracle drug.

Prednisone is not for every child, as Drachman emphasized. In his own study involving 16 youngsters, treatment was discontinued when the child finally became wheelchair bound. That rules out a large proportion of current DMD patients who already are in wheelchairs.

This steroid drug, as Drachman stressed, also whets the appetite of children for whom weight control is crucial. The muscles of a DMD patient are inexorably wasting away at the very same time when rapid growth is taking place. Adding poundage merely worsens the body's condition.

And the treatment regimen itself is rough: 16 bitter pills every other day -- a hard routine for any child. Marion Bailey, mother of Stephen, 21, and Thomas, 19, recalled having to coat the medication with applesauce so her sons could get it down.

Add to all this the time and trouble involved in repeated visits to the medical center -- often many miles from home -- for checkup and follow-up, and the treatment poses a strain on any family.Would they do it again? Absolutely, all three Baileys said, although the mother acknowledged that weight control was a major problem throughout eight long years of treatment.

For the boys, one now through community college and seeking work as a computer-aided draftsman and the other majoring in broadcast journalism, the ordeal was worth it because, as Thomas said: "I had that extra three years walking; I had a little more independence. And I think that being older when I was in the chair {12 instead of 9}, I could handle it a little better."

Stephen, a year and a bit older than Tom, said that what he wants for the future is "just . . . to live like a normal person."

Whether the actual life span of a DMD patient is extended by prednisone remains to be seen. It has been 15 years since Drachman's trials began, and all 16 of his patients are still alive. Only now are the oldest of them approaching their twenties.

William Hines is a Washington writer.