A questionnaire survey of 1005 consecutive attenders at four outpatient clinics yielded 117 (12%) budgerigar fanciers (exposed to budgerigars- known in North America as parakeets-for at least three months) and 296 (29%) former fanciers. Twnety had precipitins to budgerigar serum or droppings or both, and 10 of these together with 39 precipitin-negative patients reported undue breathlessness on exertion during exposure to buderigars. These 59 patients were investigated further, seven completing a series of inhalation provocation tests with budgerigar antigens designed to confirm or exclude budgerigar-fancier's lung (BrFL). Typical positive responses were obtained from four current and one former fancier. The prevalence of confirmed BrFL among the 11n current budgerigar fanciers was 3.4% (four cases). This was biased, however, by the inclusion of one patient whose attendance at the surveyed clinic was attributable to the disease. With the exclusion of this patient, confidence limits suggested that the true prevalence of BrFL among current budgerigar fanciers in the general population lies between 0.5% and 7.5%, which is similar to the prevalence of farmer's lung in farm workers. In view of the enormous population at risk, however, this implies that BrFL rather than farmer's lung is by far the commonest type of allergic alveolitis in Britain.