This population-based prospective cohort study compared the risk of recurrent infections in children attending family day care (< or = 3 children per family), small (10-20 children) day-care centers (DCCs), and large (> or = 40 children) DCCs. The parents of a total of 1,242 children participated in the study (97% of the families initially contacted). An infectious episode was defined as the acute occurrence of a new symptom lasting for at least 48 h and resulting in specific treatment. Two episodes were counted as such only if they were separated by a symptom-free week. Surveillance was under the responsibility of a nursing director and was similar for all three types of DCCs. During the 8.5-month follow-up period, 3,639 infectious episodes were recorded. Compared to those in family day-care, children attending small DCCs presented a higher risk for > or = 6 total infectious episodes [odds ratio (OR) = 2.4; 95% confidence interval (CI) = 1.6-3.7]; > or = 5 upper respiratory tract infections (OR = 2.2; 95% CI = 1.4-3.4); > or = 2 episodes of otitis media (OR = 2.6; 95% CI = 1.0-2.6); > or = 2 episodes of conjunctivitis (OR = 4.1; 95% CI = 2.1-8.2); and > or = 2 episodes of croup (OR = 4.1; 95% CI = 1.6-10.9). The risk for children attending large DCCs was intermediate between those in family day care and those in small DCCs.(ABSTRACT TRUNCATED AT 250 WORDS)