We examined the reliability of resistance (R) and body composition estimates measured by bioelectrical impedance analysis (BIA) in 25 women during their menstrual cycle. Weight and bioimpedance were measured daily for one cycle and sodium intake was assessed. The precision of bioimpedance measures was examined and subjects' average resistance, weight, Na intake, and calculated body composition during the four phases of menses were compared by using paired t tests. Significant differences were observed between phases 1 and 2 for R (p less than 0.001), weight (p less than 0.05), and fat-free mass (p less than 0.05) and differences were observed between phases 2 and 4 for R (p less than 0.05) and weight (p less than 0.05); no significant differences were observed for percent body fat. Changes in body weight (p less than 0.001) associated with Na intake explained a significant proportion of error in resistance measures. These data suggest the reliability of the BIA body composition approach during menses; small weight changes related to hydration status may, however, be a confounding influence.