Morphometric analysis of 90 intramuscular nerves from the biopsied biceps muscles of 16 cases of amyotrophic lateral sclerosis (ALS) were done and compared with controls. In all fascicles (large and small) of ALS, the total number and the numbers of large and small myelinated fibers were significantly reduced to 52, 16, and 64% of controls, respectively. A histogram of the largest fascicles revealed a unimodal distribution and a shift to the left, showing a single peak at 1 micron diameter. The results indicate proximal and distal involvement of the most remote portions of the nerves to the muscle in ALS and support previous suggestions of a neuronopathy.