Illnesses among children attending municipal day-care centers (DCCs) were followed in a prospective study in Helsinki during 1985-1986. The study comprised 1,905 follow-up years among children in 29 DCCs. The mean number of illness periods was 4.9 per follow-up year, 7.9 for those under the age of three years and 3.8 for older children. The corresponding average numbers of days of illness were 23, 39 and 17. The most common diagnoses were upper respiratory tract infections (46.0%), diarrhea (17.2%), otitis media (12.9%), eye infections (4.0%), acute tonsillitis (3.2%), and bronchitis (3.0%). The six most common diagnoses, all infections, caused 86% of periods and 79% of days of illness. The ten most common infectious diseases caused 90.9% of absence periods, surgical operations 1.8%, and injuries 0.8%. In children under three years of age, a small area and volume of a DCC, lack of fully mechanized ventilation, and lack of separate facilities were associated with a higher incidence of one or all of the six most common infections. A large number of children at a DCC and small homes were associated with a high incidence of one or all of the most common infections among both younger and older children. The effects of passive smoking, number of siblings, number of household members, and incomes of families were not statistically significant.