In 19 adult patients, during an acute asthmatic attack, the regional distribution of pulmonary arterial blood flow was measured by means of radioisotope lung scanning after the intravenous injection of iodinated I 131 serum albumin aggregated. All but one patient had focal perfusion abnormalities. In two of six patients examined during a repeat attack, the abnormalities appeared to be localized in the same regions, while in three other patients, different areas of the pulmonary circulation were affected. In the remaining patient, scans were normal during the two consecutive acute asthmatic attacks. In all six patients studied during remission, the scan pattern returned towards normal. These findings are of importance in the differential diagnosis of acute bronchial asthma, chronic obstructive emphysema, and pulmonary embolism.