We studied the natural history, the prevalence of atopy, and the frequency of systemic symptoms during attacks in 35 patients with cholinergic urticaria, the histologic condition of the eruption in seven patients (20%), and the response to intradermal injections of acetylcholine, histamine, and methacholine in 18 patients (51%). In most patients symptoms began between the ages of 10 and 30 years, persisted for many years, and caused them to modify their activities to avoid the provoking factors of exercise, emotion, and heat. The condition usually improved with time, and five patients (14%) had a spontaneous remission. Atopy, present in about 12 (34%) of the patients, was more frequent than in the general population. Systemic symptoms were uncommon during attacks, and patients had no greater responses than controls to the intradermal tests. The histologic study revealed neutrophils in and around the walls of superficial subpapillary dermal vessels.