Data from a large epidemiologic survey of Ontario children 4 to 16 years of age are presented concerning the frequency and correlates the use of ambulatory medical care services during a 6-month period in which a universal, first-dollar health insurance plan was used. Patterns of use of ambulatory medical care are described for three settings: doctor's offices, emergency rooms, and hospital outpatient departments. A group of children who are frequent users of ambulatory medical care (defined as using three or more services in 6 months) consumed nearly two thirds of all services. Two regression equations are presented—one predicting use/nonuse of ambulatory medical care and the other predicting the total number of visits for medical care. Although only a small proportion of the variance in use/nonuse and amount of use was explained, the major determinant of both ambulatory medical care use and frequency of use was the child's physical health status as perceived by the parent. Younger child, urban area of residence, the number of chronic medical problems of the child, and higher level of maternal education also contributed to the explanation of use v nonuse. Among ambulatory medical care users, high users were more likely to be described as having mental health problems and have parents who had been treated for "nerves." Family size and socioeconomic variables were not important factors in use, suggesting that universal health insurance reduces some barriers to ambulatory medical care for children.