Previous studies have shown that increased stress, as measured by life-change scales, is associated with poor performance. The present study examined the effects of stress on the performance of 86 adult male runners in a half marathon. Subjects responded to a questionnaire from which several independent variables were derived. These variables were entered in a stepwise multiple regression to predict race times. The combination of age, weekly mileage, and number of races previously entered yielded a significant multiple R. However, stress was not significantly related to race times, and subsequent statistical analyses were carried out to explain this lack of significance. It was suggested that the effects of stress on athletic performance may be reduced by physical and psychological variables associated with exercise or that race performance is too well determined by physical variables to be affected by life changes.