We undertook this study to determine the relative contributions of pelvic and lower limb motion to increases in the angle of passive straight leg raising in the sagittal plane. Seventeen healthy subjects underwent passive straight leg raising of their relaxed left lower limb while their right lower limb was held firmly against the table. Each subject's pelvis and left lower limb were marked so that cinematography could document motion of the pelvis and left lower limb during passive straight leg raising. Film analysis revealed that pelvic rotation began within 9 degrees of the beginning of passive straight leg raising and that the relative contribution of pelvic rotation to the angle of passive straight leg raising in relation to the horizontal plane increased in conjunction with the angle of straight leg raising. Clinicians using the passive straight-leg-raising test should be aware of the contribution of pelvic rotation to the angle of straight leg raising when interpreting test results.