PURPOSE: Repeated-sprint training in hypoxia (RSH) has been shown as an efficient method for improving repeated-sprint ability (RSA) in team-sport players but has not been investigated in swimming. We assessed whether RSH with arterial desaturation induced by voluntary hypoventilation at low lung volume (VHL) could improve RSA to a greater extent than the same training performed under normal breathing (NB) conditions. METHODS: Sixteen competitive swimmers completed 6 sessions of repeated sprints (2 sets of 16 x 15 m with 30 s send-off) either with VHL (RSH-VHL, n = 8) or with NB (RSN, n = 8). Before and after training, performance was evaluated through an RSA test (25-m all-out sprints with 35 s send-off) until exhaustion. RESULTS: From before to after training, the number of sprints was significantly increased in RSH-VHL (7.1 +/- 2.1 vs 9.6 +/- 2.5; P < .01) but not in RSN (8.0 +/- 3.1 vs 8.7 +/- 3.7; P = .38). Maximal blood lactate concentration ([La]max) was higher after than before in RSH-VHL (11.5 +/- 3.9 vs 7.9 +/- 3.7 mmol/L; P = .04) but was unchanged in RSN (10.2 +/- 2.0 vs 9.0 +/- 3.5 mmol/L; P = .34). There was a strong correlation between the increases in the number of sprints and in [La]max in RSH-VHL only (R = .93, P < .01). CONCLUSIONS: RSH-VHL improved RSA in swimming, probably through enhanced anaerobic glycolysis. This innovative method allows inducing benefits normally associated with hypoxia during swim training in normoxia.