The purpose of this study was to evaluate changes in lung function after running a marathon. Pulmonary function tests were administered to 8 men before, immediately after, and the day following competition (mean run time = 3 hr 30 min). Subjects completed maximum expiratory flow volume maneuvers breathing air and 80% He/20% O2. Lung volumes were determined by N2 washout and single breath He dilution. Closing volumes (CV) were determined using a single breath O2 test. Pulmonary diffusing capacity (DLCO), pulmonary capillary blood volume (Vc), and membrane diffusing capacity (DM) were measured with the single breath technique. There were no changes in lung volumes or flow rates, except for an increase in FEV1, after the marathon. The He/O2 delta Vmax50, delta Vmax25, and isoflow values were similar pre- compared to post-race. There were significant decreases, however, in DLCO, DM and increases in CV post-race. Vc remained similar to pre-race values. These results suggest that small airways obstruction does not occur after a marathon. The significant increase in alveolar-capillary membrane resistance, however, may reflect the occurrence of subclinical edema. Such a change would decrease lung elastic recoil and could explain the increase in CV.