To validate doubly labeled water (DLW) in aquatic athletes, energy expenditures (EE) were determined by energy intake/balance (IB) and DLW in free-living collegiate swimmers [n = 8, age 20.1 +/- 1.7 (SD) yr, height 186 +/- 11 cm, weight 74.1 +/- 9.3 kg] over 9 days. EEIB was determined from caloric analysis of duplicate foods to those consumed by subjects and corrected for changes in body composition across the study. For EEDLW, subjects were given 2H2O and H2(18)O on day 0 and 2H2O on day 10. Day 0 and 10 body compositions were measured by use of 2H dilution space, and elimination rates over days 1-10 were determined for 2H and 18O. EEDLW was calculated from CO2 respiratory quotient by use of a food quotient. Fat mass increased (P < 0.01) over the study period. Mean uncorrected energy intake was 3,895 +/- 621 kcal/day. When all eight subjects were compared, EEIB and EEDLW were 3,492 +/- 876 and 3,466 +/- 992 kcal/day, respectively. Data from one subject compared poorly despite repeat isotopic analyses. In the remaining seven subjects, EEIB and EEDLW were 3,444 +/- 925 and 3,556 +/- 1,025 kcal/day, respectively. These data indicate that DLW is a good indicator of EE in groups of swimmers and that energy requirements of swimmers are similar to other athletic groups.