We used a modified acid-fast stain to detect Cryptosporidium oocysts in unconcentrated stools from 824 Haitian children less than 2 years of age with acute diarrhea. Oocysts were identified in stools from 17.5% of 291 children seen at a rural dispensary and in 16.3% of stools from 533 children in an urban hospital. Children with cryptosporidiosis were identified throughout a 22-month period from October 1982 to July 1984, without seasonal predilection. Cryptosporidiosis was twice as common in infants greater than 6 months of age than in younger infants. Cryptosporidium oocysts were rarely found in stools of infants receiving only breast milk. The clinical presentation of children with cryptosporidiosis was similar to other patients with diarrhea who did not have cryptosporidiosis. No asymptomatic Cryptosporidium infections were detected among 71 family members of 31 children with cryptosporidiosis. Three of the 71 family members less than 3 years of age were found to have diarrhea and Cryptosporidium oocysts. In follow-up studies of 31 patients, oocysts persisted in stools for a mean of 14 days (range 5-31 days). Duration of oocyst excretion was directly correlated with duration of diarrhea. These findings suggest that Cryptosporidium is an important cause of acute self-limited diarrheal illness to Haitian children.