The authors aimed to analyze the relation of psychological predictors with medical and psychological therapy success indicators in continuous subcutaneous insulin infusion (CSII). Besides blood glucose control as a medical indicator of therapy success (by means of HbA1c levels), the authors assessed treatment satisfaction, depressive symptoms, and quality of life among 51 adult patients on CSII. The authors examined the following psychological factors that were assumed to be associated with outcome variables: coping style, locus of control, and self-efficacy. Self-efficacy correlated positively with all psychological outcome variables. A high external locus of control was related to negative glycaemic outcome and more depressive symptoms. Multiple linear regression analyses revealed that when considering all psychological factors simultaneously, locus of control was the strongest predictor of HbA1c. For the psychological outcomes, self-efficacy was the most powerful predictor. The authors demonstrated that medical and psychological diabetes-related outcomes were strongly related to psychological factors in adult CSII patients. Patients who felt responsible for the process of CSII therapy showed more positive outcomes; therefore, psychological care can be seen as an important part of diabetes education in CSII therapy.