The concept of aetiology of mental illness in 134 Malay patients was investigated by means of a 20-item checklist. About 53% of the patients attributed their illnesses to supernatural agents. Witchcraft and possession by evil spirits were regarded as common causes of illness. The number of patients who believed in supernatural causes of their mental illness was significantly higher among those who had consulted bomohs (Malay traditional healers) than among those who had not consulted them. The belief that mental illness is caused by supernatural agents is firmly held by bomohs, who reinforce this notion in those who seek their advice. Belief in supernatural causes of mental illness was not significantly associated with age, gender, level of education or occupation of the patients. Patients who believed in supernatural causes of mental illness were also found to show poor drug compliance, and the number of such patients at 6 months follow-up was significantly lower than the corresponding figure for those who did not believe in supernatural causes. The importance of understanding the patients' cultural background when treating psychiatric patients is highlighted.