To test whether congenital nemaline myopathy (NM) might have a neurogenic basis, we evaluated the number and frequency distribution of L5 motoneuron cell bodies and their myelinated ventral root (VSR) axons in a typical case. Results were compared to those from three age-matched controls and those from a child with Werdnig-Hoffmann disease (WHD). Diameter histograms of cell bodies of L5 motoneurons from controls contained three peaks which were identified as large, intermediate, and small cytons (LC, IC, and SC). In previous work from our laboratory, the numbers of LC and IC of the L5 segment was found to correspond closely enough to the numbers of LA and IA (large and intermediate diameter myelinated axons) of L5 ventral spinal root to indicate that the majority of LC are alpha and the majority of IC are gamma motoneurons. The number of LC of L5 reference cords varied from 3508 to 4916, and of IC from 967 to 2204. The diameter histogram of cytons from the NM case differed from controls; the LC and IC peaks were displaced to smaller diameter categories, but their number was normal (4520 and 2023, respectively). In WHD, the numbers of LC and IC were 211 and 946, respectively. Axons were similarly affected. The severe decrease from normal in the number of large motoneurons in WHD, and the reduction in size without loss of motoneurons in NM, indicates a different morphologic basis for NM and WHD. In NM the reduction in size without decrease in number of motoneurons is probably secondary to congenital myopathy and does not provide evidence for a neurogenic etiology.