The diameter histograms of cell bodies (cytons) in motor neuron columns at the L5 segment of the spinal cord of adult man reproducibly yield three peaks of increasing height: small (Cs), intermediate (Ci), and large (Cl). Histograms of L5 myelinated axons obtained from the ventral root have two peaks of increasing height: intermediate (Ai) and large (Al). In histograms prepared from seven cases of amyotrophic lateral sclerosis (ALS), the Cl and Al peaks were decreased selectively and severely. This provides evidence for alpha, but not gamma, motor neuron vulnerability. The Cl peak of spinal ganglion neurons and the Al peak of dorsal roots were significantly reduced in number, without a concomitant increase in Ci, Cs, and Ai peaks. This, plus earlier reports of abnormal cutaneous sensation thresholds, abnormal rates of fiber degeneration in cutaneous nerves, and dorsal column demyelination, provides evidence that large afferent neurons are affected in ALS, but to a lesser degree than alpha motor neurons.