In brief: To examine the relationship between actual performance times and predicted marathon times calculated from running velocity at the anaerobic threshold (AT), 18 male subjects ran on a treadmill to volitional fatigue. The AT was determined by examining the excess CO2elimination curve. The subjects' average velocity at the AT was 9.22 mph. The average predicted time was 2:53:49, while the average marathon time was 2:52:06. There was a highly significant correlation between the predicted and actual marathon times (r =.94, p<.01). This relationship suggests that running velocity at the AT may be critical in determining efficient running speed during marathons.