The present study was conducted 1) to determine if the use of residual volume (RV) or functional residual capacity (FRC) in underwater weighing yields any difference in body density measurements, and 2) to compare reliability of values of body density which were obtained by underwater weighing at FRC and those obtained by underwater weighing at RV. Subjects were 52 healthy young men. Thirteen of those were retested in order to assess the reliability of values. The results obtained were as follows: (1) The correlation coefficient between the determinations of body density by underwater weighing at RV and FRC was r=0.87. Small but statistically significant differences in body density were observed with the use of RV vs FRC (p<0.05). (2) The correlation coefficients between the two determinations of lung volume were r=0.841 and r=0.914 for RV and FRC, respectively. Statistically significant differences between the two values were not found for either RV or FRC.b (3) The correlation coefficients between the two determinations of underwater weight were r=0.884 and r=0.896 at RV and FRC, respectively. Statistically significant differences between the two values of underwater weight were not found for either RV or FRC data. (4) Test-retest reliability values for body density were r=0.915 and r=0.906 at RV and FRC, respectively. Further, the mean difference of the two values for both RV and FRC data was not statistically significant. Results from this study demonstrated that the values of body density obtained by underwater weighing at FRC correlated highly with those obtained by underwater weighing at RV and that they were reliable.