The clinical and laboratory features of 18 adult pellagrins are reviewed. Only four patients (22%) had the full trial of dermatitis, diarrhea and dementia. Dermatitis alone occurred in six(33%), dementia in five(28%) and dermatitis and diarrhea in three(17%). In one patient, dermentia was the initial sign of a relapse. Steatorrhea was found in six patients and was usually associated with marked alopecia. Edema without evidence of cardiac failure was present in seven patients. A diffuse increase in slow wave activity on the electroencephalogram was characteristic in patients with dementia. Fever occurred in 14 patients, and an infection was documented in 10 of these. Common laboratory abnormalities included a normochromic, normocytic anemia, lymphopenia, eosinopenia hyperuricemia, and low serum levels of albumin, urea, cholesterol, carotene, potassium, calcium, and magnesium. Adrenal and thyroid function were normal, but a low serum T4, high serum free T4, and an elevated T3 resin uptake were frequently observed. These abnormalities were corrected with treatment of the underlying nutritional disorder. In two patients initially treated with thiamine alone, and in one who received inadequate amounts of niacin and protein, there was marked deterioration of mental function, which responded to administration of niacin and proper diet.