We examined all the morbidity experienced in a six-year period by a total of 2591 children who were continuously enrolled in a prepaid medical plan. The children had received 19,291 diagnoses, each of which was assigned to one of 14 types of morbidity, and the frequency of each type was determined. Although the typical child had at least one problem in 5 of the 14 types of morbidity in the six-year period, over 20 per cent of children had at least 8 different types of problems during that time. Children with a greater variety of acute problems and more frequent acute problems were also more likely to have nonacute problems during the six-year period. Conversely, children with nonacute problems had more acute problems than other children. Our findings indicate that morbidity, like use of health services, occurred in clusters in this population of children. Therefore, an understanding of the cause and projected outcome of morbidity among children will be incomplete if the focus is only on specific diseases or specific types of illnesses.