Summary An account is given of the cutaneous manifestations of tuberculosis, sarcoidosis and polyarteritis nodosa. Skin changes in tuberculosis may be due to the presence of tubercle bacilli living in the skin or they may be secondary to disease elsewhere in the body. In the first group the following conditions are found: primary tuberculosis of the skin, miliary tuberculosis, lupus vulgaris, tuberculosis verrucosa cutis, scrofuloderma and tuberculosis cutis orificialis. The second group consists of erythema nodosum, erythematous tuberculide, Bazin's disease, papulo-necrotic tuberculide, lichenoid tuberculides, lupus miliaris disseminatus faciei and rosaccous tuberculide. In sarcoidosis skin lesions take on a variety of forms, the principal ones being classified in one of four main groups: Boeck's sarcoid, lupus pernio, erythematous sarcoid and the subcutaneous lesions of Darier-Roussey. Lesions in Boeck's sarcoid may be either nodular or in plaques and they are often found associated with lesions of lupus pernio. In polyarteritis nodosa, skin changes are found in about 25 per cent. of all cases. The lesions may be non-specific and consist of urticaria and various forms of erythema or they may be due to damage to blood vessels in or near the skin. Nodules are the most characteristic of these changes, but h˦morrhages, livedo reticularis and ulcerations also occur. Occasionally polyarteritis nodosa is confined to the skin and in these cases livedo reticularis and ulcerations are the characteristic features.