One hundred patients aged 5 through 20 years with cystic fibrosis, their parents, and their physicians were interviewed to assess patients' perceptions of compliance with prescribed treatments. Measures included self-administered questionnaires and the Medical Compliance Incomplete Stories Test. It was hypothesized that age-related differences in perceived compliance would be found and that such perceptions would vary as a function of four sociobehavioral factors (perceived severity of illness, optimism, level of independence in medical therapy, and degree of knowledge/understanding of illness). In addition, it was anticipated that the relationships between perceived compliance and the sociobehavioral factors would vary with age. Support for each hypothesis was found. Perceived compliance was related to age, with younger children showing greater perceived compliance to the medication regimen. Better perceived compliance was found to be associated with higher levels of optimism and child knowledge of the disease. Age differences in relationships between perceived compliance and optimism and child knowledge were found. Developmental implications for practice and future research are suggested.