Doctors in New York and California have diagnosed among homosexual men 41 cases of a rare and often rapidly fatal form of cancer. Eight of the victims died less than 24 months after the diagnosis was made.

The cause of the outbreak is unknown, and there is as yet no evidence of contagion. But the doctors who have made the diagnoses, mostly in New York City and the San Francisco Bay area, are alerting other physicians who treat large numbers of homosexual men to the problem in an effort to help identify more cases and to reduce the delay in offering chemotherapy treatment.

The sudden appearance of the cancer, called Kaposi's Sarcoma, has prompted a medical investigation that experts say could have as much scientific as public health importance because of what it may teach about determining the causes of more common types of cancer. First Appears in Spots

Doctors have been taught in the past that the cancer usually appeared first in spots on the legs and that the disease took a slow course of up to 10 years. But these recent cases have shown that it appears in one or more violet-colored spots anywhere on the body. The spots generally do not itch or cause other symptoms, often can be mistaken for bruises, sometimes appear as lumps and can turn brown after a period of time. The cancer often causes swollen lymph glands, and then kills by spreading throughout the body.