Allergic bronchopulmonary aspergillosis is a relatively common complication of asthma in the United Kingdom. The results of a comprehensive study on 111 patients on whom this diagnosis was made are presented. The diagnosis, which may be suspected on clinical or radiological grounds, is established by immunological methods. Evidence is sought of Type I reagin mediated reaction and Type III (Arthus) precipitin mediated reactions. It is postulated that the mechanisms underlying the various clinical and radiological manifestations of the disease are based on these reactions. The treatment of allergic bronchopulmonary aspergillosis is unsatisfactory. Disappointing results were obtained on hyposensitization. Corticosteroids are usually required to control symptoms in this disease. Further studies are required to obtain more detailed information on many of the features of allergic bronchopulmonary aspergillosis. Longterm follow-up of patients is necessary to establish the course and prognosis of this disease.