In amyotrophic lateral sclerosis (ALS) sensory function has been considered to be normal on a clinical basis. There are, however, several pathologic reports indicating that peripheral sensory fibers are involved in ALS. To evaluate changes in sensory nerve function quantitatively, we measured compound nerve action potentials (CNAP) of the median nerve by means of intraneural microneurography (MNG) and compared the results between 16 patients with ALS, age-matched 13 patients with Parkinson disease (PD) and 13 healthy controls. A tungsten microelectrode with a tip diameter of 1 micron was inserted percutaneously into the median nerve trunk at the elbow. With supramaximal electric stimulation on the median nerve at the wrist, the largest CNAP was recorded. The configuration of the CNAP was similar among three groups, consisting of a large triphasic wave followed by small multiphaic waves. In ALS patients the peak to peak amplitude (Amp) of the triphasic wave was 189.0 +/- 49.8 microV (mean +/- SD), which was significantly smaller than 273.1 +/- 90.0 microV in PD patients and 325.4 +/- 81.0 microV in normal controls (p < 0.01). The maximal nerve conduction velocity (NCV) in ALS was also reduced when compared with that in PD patients and in normal controls (p < 0.05). The average reduction in Amp in ALS was 58% of that in normal controls, while decrease in NCV was less apparent than Amp and 96% of normal controls. The Amplitude of nerve action potentials is considered to represent the density of large myelinated fibers more than 7 microns around the microelectrode.(ABSTRACT TRUNCATED AT 250 WORDS)