The incidence of respiratory tract disease was investigated in three groups of Swedish children: those in 14 day-care centers with 18 to 68 children each; those in home care (usually no siblings); and those in family day-care homes (average, four children). In family day-care homes a mother cared for her own and one to four other children during the day. A preliminary nine-month study of 41 preschool children attending a day-care center and 41 comparable children in home care showed that children under 2 years of age in the center had more days with respiratory symptoms and more febrile illnesses (four per child) than those in home care (one per child). In a subsequent eight-month study of children under 2 years of age, children in day-care centers and home care were compared with children in family day-care homes. The 108 children in centers had more febrile illnesses (five per child) than the 57 children in home care (two per child), but the 42 children in family day-care homes had as many illnesses as those in day-care centers. The data suggest that increasing the number of contacts of an infant in day care beyond four to six children does not increase remarkably the incidence of respiratory tract disease.