The purpose of this study was to examine the relation between body composition and athletic performance of long distance relay runners in a four week weight reduction program. Six male collegiate runners, aged 19.0 ± 0.9 years, participated in this study. Body water content and fat free mass (FFM) were measured by bioelectrical impedance spectroscopy.The weight loss by 4.1% of the initial body weight consisted of decreases in FFM and fat mass (54% and 46% respectively). Percentage of body fat did not change significantly by the end of the fourth week. The total body water (TBW) loss comprised of intracellular water (ICW) and extracellular water (ECW) at the end of the fourth week, and TBW loss and FFM loss were nearly the same values. Isokinetic muscular strength of right thigh, maximal oxygen intake (VO₂max) and total treadmill running time (maximal workout time) measured did not show any significant change. These results indicate (1) half of the weight loss consisted of decrease in FFM, (2) TBW loss was due to the reduction of ICW and ECW, (3) the content of FFM loss was considered to be body water, and (4) there was no impact on muscular strength of right thigh, VO₂max or maximal workout time.