The symmetry index (SI), as one of methods to evaluate gait pattern in patients with leg-length discrepancy (LLD), helps to estimate the acceptable range of inequality and to determine symmetry in the kinematic and kinetic data before and after a heel lift, although this parameter has a large standard deviation that undermines its accuracy. Thirty patients with LLD were studied by a motion-analysis system and a force plate. Joint motion of the lower extremity in the sagittal plane, back movement in the coronal plane, and three-dimensional ground-reaction forces (GRFs) were registered. From a linear-regression analysis, a mean value of inequality of 2.33 cm (range, 2.12-2.54) was found to correspond to an acceptable gait symmetry. After a heel lift, the SI of the pelvic tilt at midstance and of the vertical GRF at initial contact increases significantly, but the SI of the medial GRF at terminal stance decreases. Patients with an inequality of a mean value of 0.51 cm determined by palpating bilaterally the top of the iliac crest (the TIC1 subgroup) showed a lesser value of the SI of the center of pressure in the forward direction during stance compared with the group with a mean value of inequality of 1.39 cm (the TIC2 subgroup). As a result of our findings, we conclude that the effect of the amount of correction by a heel lift on gait symmetry is unpredictable.