Respiratory tract infections (RTIs) in small children account for a considerable proportion of health care expenditure. In 113 children, followed for the first three years of life, we studied the frequency of acute RTI and its relationship to the factors: type of day-care, age, sex, family size, living conditions, allergic predisposition, family smoking habits, and season. To elucidate the influence of age, the frequency of acute RTI and its relationship to type of day-care was longitudinally studied on a quarterly basis. The frequency of acute RTI diagnosis increased gradually from birth culminating in a peak at the beginning of the second year. Besides age and season, type of day-care was the only factor studied to show any relationship with the frequency of acute RTI diagnosis. Up to the age of almost 2 1/2 years, children attending day-care centres accounted for more RTI diagnoses than did those in home care or family day-care, categories with comparable frequencies.