Adolescents' health attitudes and adherence to treatment for insulin-dependent diabetes mellitus (IDDM) were evaluated using the protection motivation theory (PMT). We expected cognitive appraisals of adherence (self-efficacy for treatment management, response efficacy of treatment, response costs of adherence) to be more influential for adherence than appraisals of nonadherence (rewards of nonadherence, perceptions of the risks of nonadherence, perceived severity of the risks). Adolescents (N = 101) with IDDM completed self-report measures of treatment adherence and of the PMT variables. Hierarchical regression analyses revealed that cognitions concerning adherence explained a statistically significant proportion of the variance in treatment adherence (sr2 = .17). Response costs of adherence produced the strongest correlations with overall adherence and with three of the four individual components of IDDM treatment (insulin injections, blood glucose monitoring, diet). The findings suggest that persuasive health communications might focus on appraisals of adherence rather than on risks of nonadherence.