This article reviews research that suggests a relationship between diet and psychological symptoms. Mind-body dualism (as it relates to clinical practice) and the limited role of nutrition in mainstream biomedical training and treatment are discussed as background issues. Two areas of inquiry that have generated relevant research findings in this area are reviewed: (1) orthomolecular theory and vitamin deficiencies, and (2) clinical ecology/environmental medicine theory and the impact of "food allergies." Although clinical case reports and promising research findings have been reported, the impact of diet on psychological health is neither widely accepted nor integrated into mental health treatment methods. Ongoing research findings in brain biochemistry and psychoneuroimmunology point to communication pathways that can provide a clearer understanding of the links between nutritional intake, central nervous system and immune function, and psychological health status. These findings may lead to greater acceptance of dietary treatment approaches among health practitioners addressing psychological disorders.