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Thenar motor neuropathy is a rare, pure motor neuropathy of the median nerve with delayed motor latency in electrophysiological studies. We present an extremely rare case of idiopathic thenar motor neuropathy with complete median motor conduction block at the carpal tunnel level, which was confirmed by ultrasonographic and surgical findings. Right median nerve conduction studies demonstrated normal sensory responses and no motor response in the abductor pollicis brevis (APB). Needle electromyography of the right APB showed no recruitment of motor unit action potentials. Ultrasonographic examination demonstrated enlarged and swollen median nerve fascicles within the carpal tunnel. A carpal tunnel release operation was performed; the median nerve underneath the transverse carpal ligament was swollen and blanched due to decreased vascularity, suggesting the presence of compression. Pain in the thenar area improved 2 weeks after surgery. Herein, we emphasize the usefulness of ultrasonographic evaluations in determining the level of the nerve injury in cases with doubtful electrophysiological results.