The purpose of this paper is to describe the background and methods of a prospective study of medical care utilization and morbidity in a fixed cohort of over 500 preschool children whose families belonged to a prepaid group practice affiliated with Yale University. Following baseline interviews with their mothers, study children were followed for 12 months between 1981 and 1982. Information concerning the subjects' contacts with the health care facility serving members of the group practice was collected from accompanying adults, attending clinicians, and medical records. Using these data, we were able to identify episodes of care, linking all clinical visits and phone calls for single occurrences of an illness or injury. The major aim of the study is to identify psychosocial determinants of pediatric utilization (for both acute and preventive care) and of childhood morbidity. The possible predictors of principal interest are factors associated with the family environment, such as social stress and strain, family structure, and different aspects of the mother's social network. The dual emphasis on both illness and behavior outcomes is based on the important interrelationship between epidemiologic and health services research, especially when examining psychosocial effects.