A case-control study of risk factors for acute diarrhea was conducted among children under 3 years of age attending a health maintenance organization clinic in Houston, Texas. During a 19-month period from September 1985 through March 1987, 339 children with diarrhea and 363 age- and season-stratified controls were enrolled. A total of 90% of cases were under age 2 years. Compared with children cared for at home, the risk of clinic visits for diarrhea was significantly greater for children receiving child day care and was similar for those attending day care centers (odds ratio (OR) = 2.4, 95% confidence interval (CI) 1.6-3.7), day care homes (OR = 2.0, 95% CI 1.3-3.1), mother's day out (OR = 1.8, 95% CI 0.8-4.2), or when cared for by a relative (OR = 2.0, 95% CI 1.2-3.2). Rotavirus-positive diarrhea was also significantly greater in child day care (OR = 2.4, 95% CI 1.1-5.1). The day care-associated risk was highest during the first month of enrollment (OR = 3.1, 95% CI 1.8-5.4). In this population, where 40% of children receive child care, 19% of the clinic visits for acute diarrhea were attributable to child care. These data indicate that child day care increases the risk of acute diarrhea, whether in a center or in a home, and the risk is highest in the first month of enrollment.