In this study, Social Learning Theory was used to generate psychosocial predictors of regimen adherence among persons with insulin-dependent diabetes mellitus (IDDM). Four categories of predictors were formed: knowledge, expectancies, skills, and environmental support. Persons with IDDM (84 adults and 23 adolescents) completed measures within each predictor category on two occasions, separated by 6 months. Multimethod assessment procedures (self-monitoring, interviews, mechanical devices) were used to measure adherence to four aspects of the IDDM regimen: insulin injections, glucose testing, diet, and exercise. Bivariate analyses demonstrated that measures of expectancies and environmental support were reliably related to several adherence behaviours. In particular, self-efficacy expectations were related to adherence across almost all regimen areas for both adolescents and adults. Skills were also important, though only for adolescents, whereas regimen knowledge was unrelated to adherence for either adults or adolescents. Multivariate analyses showed that expectancies were consistently predictive of adherence and that, in general, psychosocial variables were better predictors of insulin administration and glucose testing than of diet or exercise. The implications of these findings for diabetes education programs are discussed.