This study assessed the accumulated oxygen deficit (AOD) during 50, 100, and 200 m front-crawl performances to compare the responses between sexes and analyse the effect of lean body mass (LBM). Twenty swimmers (16.2 ± 2.8 years, 61.6 ± 7.8 kg, and 48.8±11.2 kg LBM) performed 50, 100, and 200 m to determine accumulated oxygen uptake (V̇O2Ac). The swimmers also performed an incremental test from which five submaximal steps were selected to estimate the oxygen demand (V̇O2demand) from the V̇O2 vs velocity adjustment. V̇O2 was sampled using a gas analyser coupled with a respiratory snorkel. AOD was the difference between V̇O2demand and V̇O2Ac, and LBM was assessed by dual-energy absorptiometry. A two-way ANOVA evidenced an AOD increase with distance for both sexes: 19.7 ± 2.5 vs 24.9 ± 5.5, 29.8 ± 8.0 vs 36.5 ± 5.8, and 41.5 ± 9.4 vs 55.2 ± 11.9 ml×kg−1, respectively for 50, 100, and 200 m (with highest values for females, P < 0.01). Inverse correlations were observed between LBM and AOD for 50, 100, and 200 m (r = -0.60, -0.38 and − 0.49, P < 0.05). AOD increased from 50 to 200 m distances for both sexes. However, females presented higher AOD values, which probably relates to the effect of reduced LBM on increasing the rate of oxygen deficit (i.e., anaerobic energy demand).