The economic consequences of illnesses among children in day-care centers (DCCs) were estimated in Helsinki during a one-year period, 1985-1986. The municipal DCCs took care of altogether 14,882 children at the end of 1985. In terms of money value in 1990, the total loss due to illnesses among all DCC children was $22,485,000 (1 US dollar = 3.80 FIM). Nearly half (46%) of this loss was caused by illnesses among children under three years of age, although these represent only 20% of the children in day care. The losses were $3,535 per child place in use among children under three years of age and $1,012 among older children. Economic losses due to the guardian's absence from work totalled $1,623, deficient utilization of DCCs $1,254, visits to a physician $169, hospitalization $221, paid care of children at home $219 and antibiotic courses $12 per child place in use for those under the age of three years. The corresponding figures for older children were 541, 248, 95, 51, 73 and 4 dollars per child. More than 90% of illnesses and costs were caused by infectious diseases. Because the costs due to illnesses among children under three years of age were more than three times as high as those for older children, preference should be given to home care and family day care for younger children and efforts should be made to find out how to build healthy DCCs for them.