To determine the prevalence, incidence, and course of secretory otitis media in an age-group population, 404 3-year-old children in a town in Denmark were examined at 6-, 8-, and 12-week intervals over a 6-month period. At each of the four examinations approximately 20% of the children had middle ear effusion (MEE); it was found at least once in 42% of the children, and 6% had persistent effusion at all four examinations. Average duration of MEE was 3 months. Since spontaneous recovery rarely occurred when MEE had been present for more than 3 months, active intervention is recommended after the condition has been followed for this length of time. The highly fluctuating course of MEE demonstrated in early childhood has important implications for concepts in pediatric middle ear disease and treatment.