One hundred consecutive outpatients with symptoms suggestive of asthma who came to the Pulmonary Function Laboratory for a methacholine challenge test were studied. In addition to the forced expiratory maneuvers, forced inspiratory maneuvers were performed before and after the maximal response to methacholine. In 24 patients, the methacholine challenge suggested that they had asthma (forced expiratory volume in 1 s [FEV1] decrease greater than or equal to 20 percent). Six of these 24 patients also had a decrease in maximal forced inspiratory flow (FIFmax) greater than or equal to 20 percent and nine had a decrease in forced inspiratory flow at 50 percent of vital capacity (FIF50) greater than or equal to 20 percent, suggesting that bronchoconstriction can cause decreased inspiratory as well as expiratory flows. In 76 patients, the methacholine challenges were "negative" (FEV1 decrease less than or equal to 20 percent), suggesting that they did not have asthma. Nevertheless, in 11 of these 76 patients the FIFmax decrease was greater than or equal to 20 percent, and in 14 patients the FIF50 decrease was greater than or equal to 20 percent, suggesting that intermittent central airway obstruction is responsible for these patients' symptoms.