The folk medical system of low-income black Americans is described, from an ethnographic study of a black neighborhood in Tucson, Arizona. Comparable beliefs among Mexican-Americans, Puerto-Rican Americans, and Southern whites are traced, mainly from published sources. The system is a composite of rare elements of African origin, survivals from the folk and formal medicine of a century ago, and selected beliefs from modern scientific medicine. It includes beliefs about the prevention of illness, the classification of illnesses into "natural" and "unnatural" categories, home remedies and preventives, and the ranking of healing practitioners, according to the perception of their ability, their modes of curing, and the types of illnesses they can cure. Folk medical beliefs are at odds with scientific medicine in many respects. Medical personnel should be aware of these differences and how they might affect patient behavior.