Seventy-eight percent of the 384 patients 2 years of age and older who were enrolled in the Cystic Fibrosis Clinic at the Hospital for Sick Children participated in this study of family adjustment to cystic fibrosis. Residence with at least one parent at the time of the survey was a condition for inclusion in the study. The effect of the disease on the family was assessed using three questionnaires: an inventory of family problems as perceived by the parents and two standardized measures of family interaction. Measures of behavior and selfconcept in each patient and sibling, physical disease parameters for each patient, and demographic characteristics of the families were examined and analyzed in relation to the responses to the questionnaires on family interaction. Families of patients with cystic fibrosis were found to function very well, with normal mean scores on both standardized profiles. Children with cystic fibrosis showed more behavior problems than expected from the test norms, and healthy siblings of children with cystic fibrosis had behavior problem scores midway between those with the disease and norm scores. Measures of self-concept were remarkably similar for patients and siblings and showed levels at or above those for children and young adults in the general population. Parents psychosocial scores were influenced by the patients' physical parameters, but the severity of the disease generally was unrelated to psychosocial scores of patients and siblings. A notable exception was the correlation of emotional disturbance with amount of hospitalization in patients older than 12 years of age.