Clinical metabolic studies are being used increasingly to study the role of nutrition in cancer etiology and prevention. These studies have important applications in at least five areas. The kinetics and toxicity of potential chemoprevention agents can be investigated in preparation for intervention studies. Nutrient levels proposed as compliance markers in intervention studies can be assessed under rigorous control. Potential mechanisms of action of nutrients can be evaluated. And intermediate endpoints, markers of biologic damage, can be measured before and after controlled dietary manipulations. As a result of these contributions, clinical metabolic studies are taking on a new and important role in the interdisciplinary approach to cancer research.