Acute and persistent middle-ear effusions are the most common complications of upper respiratory illness in young children. Knowledge of the impact of day care attendance on the incidence and prevalence of these conditions is not as complete as is desirable. Several studies suggest that the incidence of acute otitis media may be higher in children who attend group and home day care, but from these studies it is difficult to assess the magnitude of the risk of otitis media related to day care attendance. Research by Danish investigators indicates that the point prevalence of middle-ear effusion may be two to four times higher in children younger than four years old attending group day care centers than in children cared for at home or in smaller home day care settings. Recent epidemiologic and laboratory investigations have begun to elucidate the role of viral infections and of virus-bacteria interaction in the pathogenesis of acute middle-ear effusion. A thorough understanding of the relationship between day care attendance and the occurrence of otitis media with effusion would require studies that quantify the differential risk of disease in different care settings and that relate illness risk to the epidemiology of both viral and bacterial infections of the upper respiratory tract in the different environments. This type of research has not yet been conducted.