A survey of the frequency of sensitization to Aspergillus antigens was conducted in a group of asthmatics in Cleveland and compared with a group of asthmatics in London, using common antigens for testing purposes. The two groups were comparable except for earlier onset, longer duration of asthma, and a larger number of males in the London group. Twenty-eight per cent of the asthmatics from Cleveland and 23% from London had immediate skin reactivity to Aspergillus. Seven and one-half percent from the Cleveland group and 10.5% of the London group had Aspergillus precipitins in the serum. Aspergillus skin test reactivity was related to the severity of airways obstruction (p less than 0.01) but was not influenced by other factors. We conclude that sensitization to Aspergillus antigens occur with equal frequency in both the United States and the United Kingdom.