Data concerning effects of the essential vitamin niacin and its active form nicotinamide were evaluated. Dietary deficiencies and excesses of these nutrients by themselves do not appear to exert any influence on in vivo carcinogenesis in animals. Varying results were produced when nicotinamide was administered at pharmacologic doses concurrently with or following carcinogen administration to mice or rats. Some investigators found significantly increased tumor formation, whereas others reported a decreased effect or no effect. Epidemiologic studies have not investigated the relationship between niacin deficiency or excess and carcinogenesis in humans.