The purpose of the present study was to examine the criterion-related validity of the sit-and-reach test (SRT) and the modified sit-and-reach test (MSRT) for estimating hamstring flexibility in children and adolescents as well as to determine whether the MSRT is more valid than the SRT. A total of 87 (45 boys and 42 girls) children (6-12 years old) and adolescents (13-17 years old) performed the SRT and the MSRT. Hamstring flexibility was measured with goniometry through the passive straight-leg raise test. Regression analysis was performed to study the association of SRT and MSRT with hamstring flexibility (criterion measure). The SRT was associated 1 with hamstring flexibility in both children (beta = 1.089, R-2 = 0.281, p = 0.001) and adolescents (beta = 6.690, R-2 = 0.333, p = 0.004). The MSRT was also associated with hamstring flexibility in both children (beta = 1.296, R-2 = 0.298, p < 0.001) and adolescents (beta = 0.588, R-2 = 0.243, p = 0.027). It is concluded that the criterion-related validity of the SRT and the MSRT for estimating hamstring flexibility is weak. The present data do not support that the MSRT is a more valid method than the SRT in children and adolescents.