From a series of 105 patients who had undergone jejuno-ileal bypass for morbid obesity, seven developed an episodic illness featuring inflammatory skin lesions, usually associated with a non-destructive polyarthritis. Tenosynovitis, myalgia and fever had also occurred. The illness abated spontaneously in four patients. The most characteristic skin lesion was an erythematous macule which developed a central vesicle or pustule. Erythema nodosum-like lesions and liquefying nodules were also seen. Early lesions were characterised by a dense accumulation of neutrophils. Changes is dermal vessels were less destructive than is usually seen in immune complex mediated vasculitis. Circulating immune complexes could be demonstrated in patients actively developing skin lesions. They were also found in some patients long after activity had ceased and in jejuno-ileal bypass subjects who had never developed inflammatory lesions. Dapsone was effective in suppressing the skin and joint symptoms when metronidazole and/or tetracycline had failed.