Twenty-one patients are described who developed asthma, or evidence of a peripheral airways reaction, while working in the electronics industry and exposed to solder flux fumes containing colophony (pine resin). This selected group contained five patients with pre-existing asthma. Fourteen patients had never smoked. Prick-testing with common environmental antigens gave one or more positive reactions in nine patients. The diagnosis was confirmed by occupational-type provocation tests in all workers with colophony-based cored solder, and reproduced by exposure to the fumes of heated colophony alone. Control subjects did not react to the test with colophony. The clinical findings and provocation test reactions have features of a hypersensitivity reaction, although so far specific antibodies have not been found.