Four patients with cold urticaria and six control subjects were studied. Gastric acid output was measured for 1 hr before and 1 hr after the immersion of both hands in ice cold water. The mean acid output fell in the control group by 16.3%. Acid output in the cold urticaria patients rose in each case following the cold challenge. In three cases it rose by a mean of 52.6% and in one case by 1707%. Histamine is released locally and systemically during the reaction of cold urticaria. This is the most likely cause of the increase in gastric acid secretion.