Forced expiratory volumes were recorded in the upright position in 10 asthmatics before and after lying supine. On consecutive days were investigated, in random order, the effect of being supine for 0.5 h, 2 h, 4 h, and of being upright as a control experiment. Peak expiratory flow (PEF) was similarly recorded in the upright position, and, in addition, at every 0.5 h in the supine position. PEF decreased progressively in the supine position, but not in the control experiment. The decrease averaged 24% when supine for 4 h. After return to the upright position, it increased rapidly but incompletely. Forced expiratory volumes tended to decrease after being supine. Thus, the supine position may induce airflow obstruction in asthmatics, and posture may be a major factor behind nocturnal asthma. This protocol may be used as a bronchial challenge test in order to study possible influences on posture-induced obstruction in asthma.