Background: Increasing importance is attributed to the knowledge that patients have concerning their illness. For psychiatric disorders, imparting information about the illness has become a standard part of treatment. Despite the great clinical relevance of knowledge about depression, only few empirical studies on this subject have been carried out. This study aimed to identify psychosocial factors associated with greater or lesser knowledge about affective disorders. Methods: Sixty-one in-patients with depression were recruited and tested with the Knowledge about Depression and Mania Inventory. Results: Almost all patients sought specific information about their disorder prior to admission to hospital. There were large differences in patients’ knowledge about the disorder and their choice of information source. Older and less educated patients had less knowledge about affective disorders. Patients with less illness knowledge also have a less favourable illness concept, poorer interpersonal relationships and more passive coping strategies. Conclusions: The results show that knowledge about affective disorders is a central illness characteristic that has numerous implications for the ability to cope with the disorder, as well as for psychotherapeutic management. The results contribute to a clarification of the relationship between psychoeducation and psychotherapy.