44 cases of an unusual condition, designated hot spring dermatitis, have been studied. Patients usually presented during the winter months with a history of having taken green sulfur spring baths within the previous 2-20 days. Skin lesions developed about 24 h after bathing and were distributed generally over the trunk and limbs, especially in the skin folds. No micro-organisms were found in either hot spring water specimens or skin lesions. Patch tests showed no positive reactions. Investigations were undertaken to determine the physicochemical characteristics of the hot spring. In its extreme acidity and high content of soluble sulfur and chloride, it differed from other nearby hot springs.