Summary and Conclusions This study of the lesions of pellagra does not give any direct insight into the origin or cause of the disease. The cutaneous lesions give no evidence of being infectious in nature. The lesions of the mouth, tongue and colon are integral parts of the disease in the light of both clinical and pathological evidence. The changes in the alimentary tract do not appear to be simple, inflammatory changes, and some underlying and contributory factor must be necessary for their production. Pellagra is a disease in which the lesions are confined to epithelial surfaces without extension to deeper tissues or organs. The cutaneous lesions are very difficult to classify dermatologically. In the evolution of the skin lesions the following stages can be recognized: The stage of injury or fibrolytic stage; reaction or the dermatitic stage; repair; compensatory vascular ectasis, or the erythematous stage; and the cicatricial or atrophic stage. The sequence and character of the cutaneous changes are consistent with the theory that they are photodynamic effects. They do, as a matter of fact, bear slight resemblance to some of the lesions of x-ray dermatitis, particularly those described by Wolbach (5). The photodynamic hypothesis could be very satisfactorily applied to the cutaneous lesions if it were not for the fact that the reaction in the tongue, pharynx and esophagus is essentially the same. Some other factor than radiant energy must be responsible for the reaction in the skin and mucous membranes. The lesions of the mouth and tongue admit of the same exact division into stages as the cutaneous lesions; injury, reaction, compensatory vascular ectasis, and atrophy. The changes described seem to furnish sufficiently definite criteria for the evaluation of results in experimental attempts at the production of the disease.