Orlando, Fla. ? Doctors were just hoping to treat symptoms when they gave people with a deadly blood disorder a drug to reduce the need for transfusions.

To their astonishment, signs of the disease itself disappeared in nearly half of them.

Specialists said the experimental drug, Revlimid, now looks like a breakthrough and the first effective treatment for many people with myelodysplastic syndrome, or MDS, which is even more common than leukemia.

“It may be, if not eradicating the disease, putting it into what I would call deep remission,” said Dr. David Johnson, a cancer specialist at Vanderbilt-Ingram Cancer Center who is familiar with but had no role in the research.

Revlimid “is not yet on the market but almost certainly will be” because of these findings, he said.

MDS refers to a group of disorders caused by the bone marrow not making enough healthy, mature blood cells. About 15,000 to 20,000 new cases are diagnosed each year in the United States, and as many as 50,000 Americans have it now. They usually suffer anemia and fatigue and need blood transfusions about every eight weeks to stay alive.

Revlimid is similar to thalidomide, a drug notorious for the birth defects it caused decades ago but that in recent years has proved effective against another blood cancer, multiple myeloma. Researchers don’t really know how it works other than that it boosts the immune system in a number of ways.

In a new study, doctors tested it on 115 people with MDS who have the most common chromosome abnormality that causes the disease.

After about six months on the drug, 66 percent no longer needed blood transfusions, said the study’s leader, Dr. Alan List of the H. Lee Moffitt Cancer Center in Tampa, Fla. A year later, three-fourths of them still don’t need transfusions.

But the big surprise was that signs of the genetic mutation fueling the disease diminished in 81 patients and vanished in 51.

“The chromosome abnormality completely disappeared, something we’ve never seen before” from a drug aimed just at boosting red blood cells, List said.