Mucocutaneous lesions are present in both acute and chronic riboflavin deficiency. The distribution of the lesions varies with the age and gender of the patient. Lesions of acute riboflavin deficiency are similar to those observed in protein-energy malnutrition of the kwashiorkor type. In chronic riboflavin deficiency the cutaneous lesions resemble monilial intertrigo and the mucous membrane lesions include a characteristic glossitis. Prompt resolution of lesions after therapeutic doses of the vitamin are given confirms the diagnosis. Biochemical changes caused by riboflavin deficiency, which explain the dermatoses and mucous membrane lesions, have not as yet been determined. Lack of information in this area is explained by the difficulty of separating cutaneous changes caused by the deficiency from those caused by trauma or other proximate etiologic agents.