The purpose of this study was to determine the effect of 100% cotton underwear worn beneath a swimsuit on the accuracy and reliability of percent body fat measurements in the BOD POD. Sixty adults (30 males, 30 females) of various ages (1887 years) and body composition (1.95-57.15%) were tested four times in one day. Subjects wore a swimsuit and swimcap for two trials and 100% cotton underwear beneath the swimsuit and a swimcap for the remaining trials. The clothing order was randomized. Thoracic gas volume was predicted. ANOVA found no significant difference between the swimsuit only trials (25.4+12.4%) and swimsuit with underwear trials (25.1+12.3%) for all subjects. Intra-class correlation coefficients were 0.994 and 0.995 for the swimsuit only and swimsuit with underwear trials, respectively. In conclusion, these results suggest that wearing 100% cotton underwear beneath a swimsuit is an acceptable practice for fitness laboratories and the reliability was very good.