['To determine how often the second lumbrical motor potential is present when the abductor pollicis brevis (APB) motor potential is absent in severe carpal tunnel syndrome (CTS).', 'Prospective study of consecutive patients with severe CTS and an absent motor potential from the APB.', 'Single-center public hospital-based electromyography lab.', 'Patients with a clinical diagnosis of CTS who had an absent median sensory response and an absent median motor response to APB on routine nerve conduction testing. Twenty-two hands of 19 patients were examined.', 'Not applicable.', 'Presence and distal latency of motor potential to the second lumbrical.', 'The second lumbrical potential was present in 17 hands (77%). The distal motor latency to the second lumbrical was prolonged in all (mean, 9.1ms; normative value, <4.1ms).'] Do second lumbrical muscle recordings improve localization in severe carpal tunnel syndrome?