From August 1 to November 1, 1984, 80 (43%) of 186 children attending a day care center (DCC) in Tulsa, OK, had diarrhea. Seventy-one percent of children less than 3 years of age had diarrhea, compared with 17% of children 3 to 8 years old. The duration of illness was 1 to 42 days (median, 8 days). At least one stool specimen was obtained from 142 (76%) of the 186 children. Children who had had diarrhea were more likely to have a specimen positive for Cryptosporidium (27 of 77, 35%) than were children who had not had diarrhea (8 of 65, 12%) (P less than 0.01). The presence of Giardia, Campylobacter, enterovirus, or rotavirus was not statistically associated with diarrhea. These results suggest Cryptosporidium as a causative agent. The secondary attack rate for diarrhea among household contacts exposed to DCC children with diarrhea was 77 of 204 (38%) compared with 25 of 273 (9%) for household contacts of children in the DCC who did not have diarrhea (P less than 0.001). Stool specimens were obtained from 79 household contacts. Six (23%) of 26 household members exposed to a Cryptosporidium-positive child had a stool specimen positive for Cryptosporidium, compared with 1 (2%) of 53 household contacts exposed to a DCC child whose stool specimen was negative for Cryptosporidium (P less than 0.01). The outbreak resolved within 2 weeks after cohorting children with diarrhea and stressing the importance of handwashing among staff and older children. cryptosporidium can cause outbreaks of diarrhea in DCC settings, can be spread within households and may be more common than previously recognized among DCC attendees and their household members.