The purpose of the present study is to assess the relationship between early acute otitis media (AOM) and exposure to respiratory pathogens mediated by siblings and day-care. A prospective cohort of 3,754 Norwegian children born in 1992-93 was followed from birth through 12 months. One or more episodes of AOM had been experienced by 25% of the children before age one. Logistic regression analysis showed that siblings attending day-care is the most important risk factor for early AOM (ORadj = 1.9 (1.4-2.3)). The total number of children in the day-care setting is another determinant for early AOM (ORadj = 2.0 (1.4-2.6) in groups of 4 or more other children and ORadj = 1.3 (1.0-1.7) in groups of 1-3 other children as compared with those who are cared for alone). Siblings who attend day-care and the number of children in the child's own day-care setting are the most important determinants for AOM the first year of life.