The incidence of cryptosporidiosis was studied in 560 cancer patients presenting with symptoms of diarrhoea. Cryptosporidium oocysts were detected in the stool specimens by modified acid fast staining. Blood was examined for HIV antibodies from patients whose stool specimens were positive for Cryptosporidium oocysts. Oocysts were detected in 7 of the 560 (1.3%) patients who were receiving chemotherapy and had diarrhoea; in five of them the symptoms were acute, while tow patients had chronic diarrhoea. Of these seven patients, five had cancer of haemopoetic system, one had cancer of cervix and one had uterine sarcoma. None of the patients positive for oocysts were positive for HIV antibodies, ruling out the possibility of an opportunistic infection due to HIV infection.