Since Maimonides, it has been common in folk medicine to proscribe milk for asthmatics because its putative stimulation of mucus production can exacerbate asthma symptoms. A literature review, however, failed to reveal any data supporting this notion. We, therefore, compared the effects of ingesting 16 oz. of whole milk (16 g lipid), skim milk (2 g lipid), and water (each on a separate day) on: (1) forced expiratory volume in 1 second (FEV1), (2) forced expiratory flow at 50% of vital capacity (V50), and (3) pulmonary diffusing capacity (DLCO) in 11 asthmatic and 10 nonasthmatic subjects. Measurements were taken at 30 minute intervals for 3 hours. The two milk types did not significantly change FEV1 or V50 in either group, indicating that the amount ingested did not change airway resistance sufficiently to alter airflow parameters. In the asthmatic group, however, DLCO decreased progressively over the 3 hours by 6.8 +/- 1.4% (mean +/- SE) per hour after whole milk (maximum reduction = 21 +/- 1.4%) but not after water or skim milk. In the nonasthmatic group, no significant effects were observed on DLCO after any of the liquids. These data suggest that milk lipids can disturb gas exchange in asthmatic patients.