This article reports the results of a qualitative field study of the ethnotherapeutic treatment practices of "curanderos," the practitioners of traditional Mexican medicine, and their effectiveness in the treatment of mental illness. Three healers and their patients from the southwestern state of Oaxaca participated in the study. The patients had a number of psychiatric disorders, including panic and dependency syndrome and schizophrenia. The evaluation of treatment practices was based on a systematic analysis of the psychotherapeutic significance of the healers' central beliefs, such as concepts of mental illness and diagnosis. The psychotherapeutic outcomes of 8 patients were evaluated in a longitudinal case study with a 6-month follow-up. 6 of the patients showed complete remission of symptoms, and 2 patients had partial remission. The results were interpreted as evidence for the clinically significant effectiveness of Mexican "Curanderismo" in the treatment of mental illness. The psychotherapeutic effectiveness could be mainly explained by specific treatment characteristics, such as the extensive use of spirituality, altered states of consciousness, and the bifocality of ritual interventions. (Contains 4 figures and 1 table.)