BACKGROUND: The results of a routine nerve conduction study (NCS) are sometimes normal in diabetic patients with clinical manifestations of sensory polyneuropathy, especially in the early stage. We report on the value of a plantar NCS in diabetic sensory polyneuropathy with normal results from a routine NCS. METHODS: Mixed nerve conduction data in the medial and lateral plantar nerves of both feet were obtained orthodromically in 47 normal subjects and 30 patients with diabetic sensory polyneuropathies with normal results from a routine NCS. The latency and amplitude of compound nerve action potentials (CNAPs) of plantar nerves were normal in the normal subjects. The results of a plantar NCS in the patient group were compared with these normal values. RESULTS: Nerve conduction abnormalities were found in 14 patients (46.7%). The rates of abnormal findings were 33.3% (20 of 60 nerves) and 31.7% (19 of 60 nerves) in the medial and lateral plantar nerves, respectively. A definite polyneuropathy pattern was found in 10 patients (33.3%). The CNAP amplitude was low or absent in 12 patients (85.7%) with abnormal nerve conduction, and 8 patients (57.1%) showed a prolonged CNAP latency. CONCLUSIONS: A plantar NCS is useful for detecting nerve conduction abnormalities in diabetic sensory polyneuropathy when normal results are obtained from a routine NCS. The diagnostic sensitivity of a routine NCS can be significantly increased by adding a plantar NCS when evaluating patients with sensory polyneuropathies, especially in the early stage.