To determine the epidemiology of otitis media (OM) during the first three years of life, physicians participating in the Greater Boston Collaborative Otitis Media Program followed prospectively from birth 2,565 children. At every visit we recorded results of pneumatic otoscopy and epidemiology data. By three years of age 71% of the children had had one or more episodes of acute otitis media including 33% who had three or more episodes. Features associated significantly with first episode of OM were: sibling with recurrent OM, race (white > black), and sex (male > female). Having a sibling with allergy disposed propositi to first episode. Features associated significantly with recurrent OM (greater than or equal to three episodes) were those noted above. A parent with recurrent OM disposed propositi to recurrent OM. Middle ear effusion (MEE) persisted for prolonged periods after OM; after the first episode of OM, 70% of children still had MEE at two weeks, 40% had fluid at one month, 20% had fluid at two months, and 10% had fluid at three months. The sole feature associated significantly with persistent effusion in the middle ear after the first episode of OM was the practice of giving a child a bottle in bed.