Adenovirus infections and respiratory illnesses among 123 children attending a research day care center were analyzed from data collected prospectively from 1967 through 1981. During the 15-year study period, 298 adenovirus isolates were recovered from respiratory secretions of the study children, and five different serotypes were identified, with types 1, 2, and 5 accounting for 92% of the typed isolates. Annual incidence of each serotype varied greatly, with types 2 and 5 predominating before 1975 and type 1 thereafter. Exposure to adenovirus in the day care center correlated with acquisition of infection. Through two years of age, children were exposed to 3.4 +/- 0.9 (mean +/- SD) different serotypes and became infected by 2.1 +/- 0.8 different types. Most infections occurred before the child's second birthday, and incidence was highest from six to 12 months of age (1.4 primary infections/child-year). Mild and febrile illness accompanied 75% of primary isolations. Otitis media occurred in 37% of illnesses over all ages and in greater than 50% of illnesses among infants.