Puerto Ricans living in urban areas of North-Eastern United States, whether patients or randomly selected residents of the community, differ very little in their conception of the causes of mental illness. They distinguish two broad categories — “craziness” (or psychotic behaviour) and “nervousness” (bad nerves), which we would categorize as psychosomatic or neurotic conditions. For both types of conditions there is believed to be a range of causative factors which can be considered either supernatural or natural. Spiritism, witchcraft, and fate are believed to be the most important supernatural causes of mental illness, and become clearly apparent as the main factors when informants are asked about them in such a way that they are not obliged to speak directly about their own personal beliefs in supernatural determinants of illness. The authors discuss the significance of supernatural beliefs in mental illness causation in the context of patients’ appeal to community folk healers or to mental health clinics.