BACKGROUND: Patients' beliefs about the cause of their depression can affect their help-seeking behavior, treatment preferences, coping strategies and treatment compliance. There are few studies exploring depressed patients' beliefs about the causes and to our knowledge none in a Swedish population. However, previous studies show that the patients more often mention environmental and psychological causes than biological. The aim of this study was to further explore depressed patients' answers to an open-ended question about the etiology of their depression. METHODS: Primary care patients, participating in a study evaluating patient education, were asked an open-ended question about their beliefs about what had caused their depression. Answers were obtained from 303 patients. RESULTS: The analysis of the patients' beliefs emerged into 16 different categories of explanations for depression that could be organized into three larger themes: current life stressors, past life events and constitutional factors. Work-related stress was the most commonly mentioned cause, followed by personality and current family situation. Only 3.6% stated biological reasons. LIMITATIONS: We could only count the frequency of mentioned causes, but no ranking of the importance of these causes. CONCLUSIONS: Primary care patients often gave multi-causal explanations to their depression. Biological explanations were rare. Their beliefs were predominantly current life stressors such as work or family situation and also their own personality. Patients' beliefs about their illness are important in the patient-doctor encounter, when developing new treatment strategies aiming at improved adherence to both psychopharmacological and psychotherapeutic treatments and also in patient education programs.