OBJECTIVE: To verify the effectiveness of nerve block to the femoral nerve (motor branch of rectus femoris) for stiff-legged gait in spastic patients and to identify factors which influence its effect. METHOD: EMG-guided femoral nerve (motor branch of rectus femoris) block by 2% lidocaine and/or 5% phenol was performed on 33 patients with stiff-legged gait: 22 were spastic hemiplegia after stroke; 10 were spastic paraparesis; 1 was spastic diplegia. Subjective improvement in gait performance was evaluated. Pre- and post-block comparisons were made by objective parameters, including gait speed and sagittal knee kinematics. RESULTS: Twenty five of all subjects showed subjective improvement in gait performance and significant differences of pre- and postblock gait data after nerve block by 2% lidocaine. Eighteen of 19 patients who had activity of rectus femoris alone at swing phase showed subjective improvement in gait performance and significant differences of pre- and postblock gait data after this procedure. Twenty two of 25 patients with sufficient strength of hip flexor showed the same result as the above. There were no significant differences between the subgroups depending on the degree of the quadriceps spasticity. 5% phenol block for more long-term effect also showed significant increase in knee flexion at swing phase in these 25 patients. CONCLUSION: Femoral nerve (motor branch of rectus femoris) block can be a effective treatment modality for stiff-legged gait. Sufficient strength of hip flexor muscle, rectus femoris activity without activities of vastus muscles at swing phase on dynamic EMG are the factors which predict the favorable outcome in this procedure. But, diagnostic block with lidocaine is a mandatory to predict its effect in clinical practice.