The efficacy of spiramycin was evaluated in a double blind, placebo-controlled study of 44 immunocompetent infants ages 2 to 13 months who had acute diarrhea caused by Cryptosporidium. Twenty-one patients received spiramycin (100 mg/kg/day) for 10 days and 23 received placebo. On admission the patients in both groups were comparable regarding demographic and clinical characteristics. The infants who were treated with spiramycin had a shorter duration of diarrhea (mean, 5.2 vs. 7.3 days; P = 0.002) and a shorter duration of excretion of oocysts in the stools (7.1 vs. 8.5 days; P = 0.032) compared with those treated with placebo. No clinical or parasitologic relapses were seen in patients of both groups. Mild adverse effects to spiramycin were observed in 2 patients (10%). Spiramycin appeared to hasten clinical recovery and decrease the duration of oocyst excretion in immunocompetent children with diarrheal illness caused by Cryptosporidium.