Lung scanning is a simple and effective way of determining regional pulmonary arterial blood flow (1, 2). Quantitative data obtained thereby agree with those procured by the more tedious differential spirometry (3). Lung scanning is uniquely suited for study of acute conditions affecting the perfusion of various regions of the lung, as the injected radioactive particles lodge within the pulmonary arterioles and capillaries during their first passage through the lungs and remain there for several hours. A scan performed within several hours after the injection of labeled particles gives essentially the same data concerning blood flow as a scan immediately after injection. Injection can therefore be performed during the acute condition under investigation and scanning carried out later under less stressful circumstances. We believed that the technic would be particularly useful for studying the pulmonary arterial circulation during acute asthma. In spite of the fact that asthma affects approximately 0.5 per cent of the general population, there is little information concerning the pulmonary arterial circulation during the acute asthmatic attack because of the difficulty in using previously available technics for measuring pulmonary blood flow. Methods Thirteen adult patients received, while seated, an intravenous injection of 300 microcuries of I131- macroaggregated albumin (MAA) during an acute asthmatic attack. Selection was on the basis of a history of recurrent acute episodes of wheezing and dyspnea with symptom-free intervals. After injection, the patients were treated with aminophylline, epinephrine, and phenobarbital and, after relief of symptoms, were scanned in a supine position. An upright postero-anterior roentgenogram of the chest was also obtained. Lugol's solution was given to prevent thyroid uptake of radioactive iodine after metabolism of the MAA particles. Four patients were re-examined during a symptom-free interval, and 2 were studied during a second acute asthmatic attack. Results All 13 patients had clear-cut focal abnormalities of pulmonary perfusion (Fig. 1, A). One of the 4 studied during a symptom-free interval showed return of the pulmonary arterial blood flow to normal and 3 exhibited marked improvement (Fig. 1, C). The 2 patients studied during a repeat asthmatic attack showed focal areas of decreased pulmonary arterial blood flow, which differed in location and degree from the previously demonstrated abnormalities. The majority of the defects were in the upper or mid-lung fields (TABLE I). The chest roentgenograms were normal in most patients. Discussion In normal persons in an upright position there is a gradual increase of ventilation and a relatively greater increase in perfusion at the bases of the lung as compared to the apices.