This study was designed to investigate the mechanisms underlying the significant reductions in vital capacity after marathon racing originally reported over 50 years ago. Spirometric, maximum expiratory flow-volume (MEFV), DLCO and residual volume (RV) measurements were made on 13 runners (11 males and 2 females) 1--2 weeks prior to the marathon, immediately after finishing the race, and again the next day. An average 0.48 L (8.6%) reduction in post-race forced vital capacity (FVC) was accompanied by an equivalent increase in RV. Thus, total lung capacity did not change, and the reduction in FVC was a result of an expiratory rather than an inspiratory limitation. Post-race FEV1 and FEF200-1200 remained unchanged while FEV1-2, FEF1 and FEF2 were reduced 19.7, 26.7 and 23.3%, respectively. Mean DLCO was unchanged. Pulmonary function on the following day was similar to that observed on the control day. These data indicated that after the race, expiratory flow was unaffected at high lung volumes, but was decreased at low lung volumes (within the effort-independent portion of the MEFV curve). The results are compatible with small airway closure occurring at an increased lung volume, which would result in a decreased FVC and an increased RV.