The ability of adolescents with insulin-dependent diabetes mellitus (IDDM) to assume responsibility for self-management is complicated by normal psychosocial developmental tasks, including establishing independence from authority. We evaluated self-managerial behavior and its relation to cognitive maturity. Forty-one adolescents with IDDM (age range 12 to 21 years) and their parents, who were trained to self-adjust insulin on compensatory and anticipatory bases, participated. The data indicated that parents withdrew from the insulin adjustment process as their adolescents grew older. Parental participation had virtually ceased by the time the child reached age 15 years. Parental withdrawal, however, was not always balanced by the adolescent's assumption of responsibility for insulin adjustments. Older adolescents were statistically no more likely than younger adolescents to self-adjust insulin doses. Both self-adjustment and metabolic control (HbA1 values) among adolescents were, however, related to cognitive maturity. More cognitively mature adolescents were also more likely to perceive themselves as being in control of their illness. Thus, older adolescents who are less cognitively mature than their peers are sometimes given responsibility for self-managerial behaviors that they are unable to assume.