Allergic bronchopulmonary aspergillosis might be less frequent in North America because the incidence of immediate sensitivity by asthmatics to A. fumigatus is less. In order to check this hypothesis, 200 asthmatics were skin tested with two extracts of A. fumigatus which had been shown to produce positive reactions in fifty patients who had allergic aspergillosis. Of the asthmatics, 21.5% reacted to the commercial extract by prick testing and 39% by intradermal testing. Using an extract kindly provided by Professor Pepys, 19.5% reacted to a concentration of 1 mg/ml and 31.5% to 10 mg/ml. By the prick method, 21.5% reacted to both extracts. Specific IgE was measured with one of the extracts and a good correlation (r = 0.48) was found with the size of the prick reaction. The increase in specific IgE was reflected in the increase of total IgE (r = 0.84). The authors conclude that the incidence of immediate sensitivity to A. fumigatus in asthmatic patients in North America is at least equal to that found in the U.K.