The purpose of this paper is to review the research on the relationships among ethnicity, culture, neuropsychiatric diagnosis, and treatment. Psychiatric nurses provide care to an ethnically and culturally diverse group of clients. Knowledge of ethnic and cultural differences are essential to diagnosis and treatment. Ethnic diversity affects psychiatric diagnosis. Cross-ethnic differences in genetics, diet, environmental exposure, and fetal, childhood, and adolescent development may result in varied experiences of psychiatric illness among ethnic groups. Ethnic diversity also affects psychiatric treatment. There are dramatic ethnic differences in the metabolism of psychotropic medications and the effects of drugs on target organs. These differences are again due to genetic variation, exposure to different diets and environments, and other medications in use. Cultural diversity influences both diagnosis and treatment. Cultural forces shape symptom formation and the expression of distress, creating many sources for misdiagnosis based on DSM-IV criteria. The culture-bound syndromes represent unique illness forms with a natural history distinct from DSM classification. Culture also influences treatment expectations, therapeutic compliance, family involvement, and the interpretation of side effects, all of which help determine whether or not treatment will be effective. Neuropsychiatric nurses can contribute to research by studying cross-ethnic differences and similarities in biological markers of mental illness. A second significant area for research is that of ethnicity and psychotropic drug metabolism and pharmacodynamics.