The purpose of the study was to quantify the relative contribution of low back flexibility and hamstring flexibility in the sit and reach test item of the AAHPERD Health Related Fitness Test in order to examine the validity of the sit and reach test. Subjects were 100 female students, 13 to 15 years of age in physical education classes. Hamstring flexibility was measured using the Leighton flexometer. Spinal mobility was measured using a tape measure. The sit and reach test was performed according to instructions given in the AAHPERD Test Manual. Data were analyzed using correlation, linear regression, and multiple regression. Conclusions of the investigation were (1) hamstring flexibility is moderately related to the sit and reach test, (2) low back flexibility has a very small relationship to the sit and reach test, and (3) the sit and reach test is an inadequate measure of low back and hamstring flexibility.