Shedding light on problems of mental health and illness that have baffled public health workers attempting to improve the health and welfare of Mexican Americans living in the lower Rio Grande Valley, this document reports the folk customs, social organization, medical practices, and beliefs of the Mexican American of this area. Chapters describe the population, social structure, values, and customs. Particular emphasis is given to the Mexican American concept of illness, the medical referral system, and "curanderismo" (the system of disease theory and curing techniques associated with the curandero whose healing powers are believed to be a gift from God). In conclusion, 14 recommendations are given for improving the relationship between modern medicine, welfare workers, and the Mexican American in need of assistance. Some of these recommendations would require a changed relationship between doctor and patient, even involving cooperation between physician and curandero. It is suggested that the United States culture has much to learn from the Mexican American citizens about family solidarity, child rearing, respect patterns, and religious values. (AN)