The skin test reactivity to allergen and histamine differs according to the age of the patients, but complete data from infancy to old age are still lacking. Three hundred sixty-five subjects (1 to 85 years of age, 33.9% atopic, and 50.1% male patients) were prick tested with threefold dilutions of histamine hydrochloride (1 to 243 mg/ml). There was a significant (p less than 0.0001; F test) main effect of age on the skin reactivity to histamine. Age groups were defined and statistical analysis were performed by means of parallel line bioassay. All dose-response curves were linear and parallel. There is a significant increase in the mean wheal size between 4 to 5 and 6 to 9 years of age, 10 to 14 and 15 to 20 years. There was almost no difference between 15 to 20 and 21 to 50 years. No difference was observed between 21 to 30, 31 to 40, and 41 to 50 years, and then, the mean wheal sizes decreased significantly to reach a plateau after the age of 60 years. There was no sex difference, and skin tests with histamine were similar in atopic and nonatopic individuals.