The limb motor cells of the C6 segment of the spinal cord were counted and correlated with quantified histological findings of biceps brachii muscles in controls and in cases of amyotrophic lateral sclerosis (ALS). In 12 controls the motor cells were divided into larger ones with a minimum diameter greater than 20 micron, and smaller cells. Total numbers of the larger motor cells decreased significantly in 11 of 12 cases of ALS and of the smaller cells in 4 cases. In 4 controls most of the constituents of the biceps brachii muscle were normal-sized fibers, while in ALS smaller fibers and interstitial connective tissue increased and hypertrophic fibers decreased in association with a decrease of normal-sized fibers. The correlation coefficients between total numbers of the larger or smaller motor cells and normal-sized fibers in ALS were 0.92 and 0.65 respectively, and the larger motor cells, correlating with muscular atrophy of the upper arm, were considered to be alpha motor cells. Although in ALS the larger motor cells decreased almost diffusely, there were segmental variations, similar to controls, in numbers of the motor cells per 500 micron thickness.