To determine the epidemiology of acute otitis media (AOM) and duration of middle ear effusion (MEE), we followed consecutively enrolled children from shortly after birth until 7 y of age. Because some children dropped out of the study, data were analyzed for 877 children observed for at least 1 y; 698 were observed for at least 3 y, and 498 were observed until 7 y of age. By 1 y of age, 62% of the children had greater than or equal to 1 episode of AOM and 17% had greater than or equal to 3 episodes; by 3 y of age, 83% had greater than or equal to 1 episode of AOM and 46% had greater than or equal to 3 episodes. The peak incidence occurred during the second 6-mo period of life. Significantly increased risk (by multivariate analysis) for AOM was associated with male gender, sibling history of recurrent AOM, early occurrence of AOM, and not being breast fed. MEE persisted after onset of AOM for weeks to months; prolonged duration of MEE was associated with male gender, sibling history of ear infection, and not being breast fed.