Morphometric assessment of the size distribution of cell bodies (cytons) in motor neuron columns from serial sections of lumbar spinal segments reproducibly yields three peaks. Using troughs between peaks as division points, it is possible to determine the number of large (Cl), intermediate (Ci), and small (Cs) cytons per lumbar segment. In a patient with above-knee amputation 4 1/2 years previously, the number of Cl was 42%, Ci 99%, and Cs 129% that of normal. Nine years after total amputation of a lower limb for chondrosarcoma, a second patient exhibited values of 25%, 14%, and 55%, respectively. Normal results were obtained from the non-amputated sides. These results, and our previous studies on the permanent axotomy model, show that loss of target tissue by axotomy leads to atrophy and then loss of motor neurons. Extrapolated to the interpretation of morphologic abnormalities of proximal neuronal alterations in peripheral neuropathy, these results suggest that atrophy and loss of such proximal elements cannot be explained by acute distal pathology, but could be the result of chronic disorders, particularly when severe.