Cryptosporidium parvum intestinal infection in immunodeficient patients can cause severe intestinal fluid losses with severe dehydration or chronic diarrhea with malnutrition. Therapies tried in human beings and animals include paromomycin, clarithromycin, azithromycin, octreotide, hyperimmune bovine colostrum, and bovine transfer factor. No specific therapy has been found to be consistently beneficial to children. We report azithromycin treatment of four children with acquired immunodeficiency syndrome who had severe diarrheal illnesses in which Cryptosporidium parvum was the sole pathogen detected. Three of these children had a marked decrease in stool volume and frequency within 36 hours of initiating therapy and resolution of diarrhea within 5 days; Cryptosporidium organisms became undetectable on examination of stool or colonic biopsy or by both after therapy was discontinued. A fourth patient required prolonged therapy with azithromycin to achieve clearance. Azithromycin therapy should be considered for immunocompromised patients with intestinal Cryptosporidium infection. (J PEDIATR 1996;129:297-300)