The complex and arduous treatments required to maintain the health and prolong the life of patients with cystic fibrosis combine with other factors to create a climate within which the active following of prescribed medical treatment is often compromised. The authors' clinical experience and collection of descriptions of more than 1,200 critical incidents from 223 patients and members of their immediate families has led to the description of three basic typologies of nonadherence or noncompliance in the medical treatment of this illness. These are inadequate knowledge, psychosocial resistance, and educated nonadherence. Clinical use of this conceptual framework may enhance diagnostic and treatment efforts. Although the specific application described is concerned with cystic fibrosis, it seems reasonable to conclude that the same principles are applicable to other chronic illnesses of childhood.