The aim of this study was to analyze the influence of hamstring muscle extensibility on the hamstring criterion-related validity of the sit-and-reach (SR) and toe-touch (TT) tests. Two hundred forty young adults (mean age: 22.9 +/- 3.6 years) participated in this study. Three trials of straight leg raise (SLR) (left and right), SR, and TT tests were performed in a random order. The subjects were dichotomized into group A (subjects with an SLR angle < 75 degrees ) or group B (subjects with an SLR angle >/= 75 degrees ). The correlation values between SLR angle and SR and TT scores were calculated and compared between both groups. Group B elicited higher mean SR and TT scores than group A (p < 0.001). Group A showed low associations between SLR with respect to the SR (r = 0.31-0.41) and TT (r = 0.28-0.40) tests. Group B showed moderate values for both SR (r = 0.55) and TT (r = 0.60-0.61) tests. The hamstring criterion-related validity of the SR and TT tests is influenced by hamstring muscle extensibility. From the results of this investigation, we find that the SR and TT tests are not valid measures of hamstring extensibility for subjects with reduced hamstring muscle extensibility.