The ventral horn cells of the fourth lumbar segment were morphometrically analysed in six cases of amyotrophic lateral sclerosis (ALS; there common forms and three pseudopolyneuritic forms), six of multiple system atrophy (MSA) with autonomic failure, four of X-linked recessive bulbospinal neuronopathy (X-BSNP), and seven age-matched autopsy cases of non-neurological disorders. In the common form of ALS, large and medium-sized neurons of the medial and lateral nuclei were markedly lost; small neurons in the intermediate zone were slightly diminished but fairly well preserved. In the pseudopolyneuritic form of ALS, marked loss was present in the large and medium-sized neurons, and in the small neurons located in the intermediate zone as well. In the MSA, in contrast to ALS, there was a marked reduction in small neurons in the intermediate zone, and large and medium-sized neurons of the medial and lateral nuclei tended to be preserved. In X-BSNP, large and medium-sized neurons were almost completely lost and small neurons were also markedly depopulated. These findings indicated that the pattern of neuron loss in the ventral horn is distinct among these diseases depending on size, location and function of the ventral horn cell population. These disease-specific patterns of neuron loss suggest a difference in the process of neuronal degeneration of ventral horn cells among the disease examined.