The clinical features of cryptosporidial infection in 20 patients with hematologic malignancies were assessed. Five patients had severe diarrhea, 10 had moderate diarrhea, and five were asymptomatic carriers of Cryptosporidium. Extraintestinal cryptosporidiosis with pulmonary involvement was observed in one case and relapse of cryptosporidiosis in four. All but one patient recovered from cryptosporidiosis whether or not spiramycin was administered. Cryptosporidiosis in T cell-depleted recipients of allogeneic bone marrow transplants did not appear more severe than that in patients undergoing autologous bone marrow transplantation or conventional chemotherapy.