Nine male patients with the acquired immunodeficiency syndrome and one female patient who had an allogeneic bone marrow transplant for acute myeloblastic leukemia in relapse developed severe debilitating diarrhea. Cryptosporidium species were found in the stools of all patients. After receiving treatment with spiramycin for 1 week, five patients had complete resolution of the diarrhea and four patients had symptomatic improvement. One patient responded gradually and the diarrhea resolved after 30 days of treatment with spiramycin.