The immediate changes in regional ventilation and pulmonary blood flow were studied in five adults with perennial asthma, who were challenged with an allergen inhalation (pollen or moulds). The studies were based on gammacamera images of 81 mKr and of 99mTC labelled particles in the lungs and the results were compared with simultaneous changes in peak expiratory flow. Moderate to profound changes were observed after antigen-induced asthma (AIA), and ventilation and perfusion defects were matched in most regions. Peak expiratory flow was reduced in most of the cases after AIA.