An objective comparison of intrathecal lidocaine versus fentanyl for the treatment of lower extremity spasticity. The reduction of spasticity after administration of intrathecal fentanyl, 35 micrograms, and intrathecal lidocaine, 50 mg, was compared with preinjection spasticity levels in ten subjects with central nervous system disease or injury. Spasticity was objectively assessed by an electronic instrument that simultaneously measures degrees of extension and force during passive knee extension. Duration of spasticity relief and adverse effects were recorded. Both drugs equally reduced spasticity and increased the range of motion. There were no supraspinal side effects from the fentanyl, whereas three subjects became hypotensive after receiving lidocaine. The reduction of spasticity with intrathecal fentanyl lasted at least 3 h. The authors postulate that fentanyl reduced spasticity by an effect on spinal pathways. Intrathecal fentanyl should be considered as an alternative to lidocaine for diagnostic blocks in patients with spasticity. 