The cytotoxic functions of NKs (natural killer cells) are critical in enabling the immune system to cope efficiently with malignancy. In the present study, we compared how various exercise regimens without/with hypoxia influence phenotypic characteristics of NK subsets and cytotoxicity of NKs to NPCs (nasopharyngeal carcinoma cells). A total of 60 sedentary males were randomly divided into five groups. Each group (n=12) underwent one of five regimens: normoxic (21% O(2)) control (N-C), hypoxic (15% O(2)) control (H-C), normoxic exercise (50% maximal work rate under 21% O(2); N-E), hypoxic relative exercise (50% maximal heart rate reserve under 15% O2; H-RE) or hypoxic absolute exercise (50% maximal work rate under 15% O(2); H-AE) for 30 min/day, 5 days/week for 4 weeks. The results showed that hypoxic exercise regimens increased pulmonary ventilation and tissue oxygen utilization. Moreover, the H-RE regimen resulted in enhanced aerobic fitness at a less intensive training workload in the H-AE regimen. Before each regimen, strenuous exercise elevated NK perforin/granzyme B content and promoted cytotoxicity of NKs to NPCs. However, the percentage of NKs expressing homing (CD11a)/terminally differentiated (CD57)/inhibitory [KLRG1 (killer cell lectin-like receptor G1)] molecules that entered the bloodstream from peripheral tissues increased following this exercise. After 4 weeks, both the H-AE and H-RE regimens produced an up-regulated expression of memory (CD45RO)/activating (NKG2D) molecules and was accompanied by a decrease in CD57/KLRG1 levels on NKs at rest and after strenuous exercise. Furthermore, the two regimens increased resting and exercise NK perforin/granzyme B content and NK-induced phosphatidylserine exposure of NPCs. In contrast, no significant change in the phenotypic characteristics of blood NK subsets or NK-induced NPC apoptosis was observed in the N-C, H-C and N-E regimens. Therefore we conclude that 15% O(2) exercise training reduces terminally differentiated NK subsets and up-regulates the expression of activating molecules and cytotoxic granule proteins in NKs, thereby enhancing the capacity of anti-NPC cytotoxicity by NKs. These findings could help to determine effective hypoxic exercise regimens for improving individual aerobic capacity and simultaneously promoting the natural cytotoxicity of NKs.