The most stubborn, heretofore incurable kind of leukemia has begun to yield to new treatments.

The most optimistic report yet on this disease -- acute myelogenous leukemia (AML), which affects both adults and children -- appears in today's New England Journal of Medicine. It offers hope that, before long, as many as a third of these patients may be saved, compared with almost none five years ago.

Leukemia is a cancer of the blood and blood-forming organs. In the early 1970s doctors began getting good results in the other main form of severe leukemia, acute lymphocytic leukemia or "ALL" for short, which mainly affects children.

Doctors were still arguing then whether they should even try to treat AML, because the treatment with with anti-cancer chemicals was rigorous and seemed almost hopeless.

The word "optimistic" still must be used cautiously. But doctors in several centers say they are encouraged by recent results.

Three Boston hospitals and Harvard Medical School report today that of 83 patients treated by combinations of powerful chemicals between February 1976 and October 1979, 41 remain alive. More important, 29, more than a third, remain free of diseasde after two to four years.

"It's beginning to look as though if you can go two years without a relapse, you have close to a 50 percent chance" of a cure, said Dr. Howard Weinstein of the Sidney Farber Cancer Research Institute, head of the Boston group.

Dr. Emil Frei, physician-in-chief at Farber, told a recent American Cancer Society seminar the results "suggest" AML "may be curable in 20 to 40 percent of patients," especially those under age 50. AML occurs more often in adults than children, but the Boston patients' ages ranged from infancy to 50.

Dr. Peter Wiernick of the National Cancer Institute said, "What we're seeing at several centers is that about seven AML patients in 10 respond quickly to treatment. In the past, most soon relapsed. Now about 25 percent of the responders are going four to five years so far disease-free."

"The important thing," Weinstein said, "is to tell people there is a reasoble chance today for long survival and patients should be referred to major medical or cancer centers for evaluation and treatment."

The best results, doctors said, still must await evaluation of several treatment plans being tried in Boston and elsewhere, including Johns Hopkins University and the cancer institute's wards at the U.S. Public Health Service Hospital in Baltimore.

The cancer institute group is using chemicals plus either removal of the spleen, a blood-forming organ, or immunological treatment -- injection of treated leukemia cells from other patients.

Some groups give some patients transplants of bone marrow, another blood-forming organ, from their healthy brothers or sisters or, sometimes, parents.

The Boston doctors first gave an intensive course of drugs, lasting three or four months. The patients then received more treatment, using various drugs in sequence, for 14 months. In this period, patients were generally out of the hospital, and often at school or work wearing a portable infusion pump, a device attached to the belt that continuously injected a chemical beneath the skin.

"Our dosages of chemicals are higher than most centers have used," Weinstein said. "What we're asking of these patients is a window out of their lives," a period that is "fairly hard" on them, after which they can return to more nearly normal life with a hope for survival.