DIABETEX is a knowledge-based, computer-supported consultation system for the therapy of type I diabetic out-patients. Three knowledge bases contain the medical and technical knowledge for treating either adults, adolescents or children by means of injections or by pumps. For the evaluation of the quality of the decision proposals three groups of patients were studied. The results obtained in test phase 1, i.e. the retrospective evaluation of DIABETEX decision proposals by the experts, and in test phase 2, i.e. the parallel work of DIABETEX and of an expert and the subsequent evaluation by experts, provide the basis for the further development and application of the system by non-expert diabetologists.