Earlier studies have shown that one in eight children persist in being high users of health care services over long periods of time and, conversely, about the same proportion of children are consistently low users. As these studies failed to discover reasons for these persistent deviations from average, this study explored three possible explanations: persistent morbidity, mental health problem, and familial patterns of use. Although all three phenomena were associated with both persistence of high utilization of services as well as an overall large number of visits, the number of types of morbidity was the most significant correlate of high use. Children who were constantly low users of services were much more likely to have only a few types of morbidity whereas children who were constantly high users were much more likely to experience a wide variety of types of problems. These findings suggest that an understanding of the demand for health services requires an understanding of the interrelationships of illnesses within individuals rather than a focus on particular illnesses or particular types of illnesses.