In the present study, to investigate the effect of glucosamine, a component of glycosaminoglycans with a chondroprotective action, on articular cartilage in athletes, we looked at soccer players, who expose their joints to excessive motion and loading, and compared the levels of biomarkers for type II collagen degradation (CTX-II) and type II collagen synthesis (CPII) between soccer players and non-athlete controls, and in soccer players before and after glucosamine-administration. CTX-II (P<0.01) and CPII (P=0.08) levels were substantially elevated in soccer players compared with those in controls, indicating that cartilage metabolism (type II collagen degradation and synthesis) is increased in soccer players. Of note, glucosamine administration (1.5 g and 3 g/day for 3 months) significantly decreased the CTX-II level (P<0.05); however, the effect disappeared after withdrawal of administration. In contrast, glucosamine administration did not essentially affect the increased level of CPII. Furthermore, cartilage damage was evaluated by using the ratio of type II collagen breakdown to synthesis (CTX-II/CPII). The ratio in soccer players was significantly higher than that in controls (P<0.05), suggesting that type II collagen degradation is relatively enhanced compared with type II collagen synthesis in soccer players than in control students. Of importance, the ratio was reduced by glucosamine administration but returned to the pre-administration level after withdrawal of administration. Together these observations suggest that glucosamine is expected to exert a chondroprotective action in athletes (soccer players) by preventing type II collagen degradation but maintaining type II collagen synthesis, although the effect is transient and disappears after withdrawal of administration.