The lumbar ventral horn cells and myelinated fibers in the ventral spinal roots of Guillain-Barré syndrome (GBS) and chronic inflammatory demyelinating polyradiculoneuritis (CIDP) were morphometrically analyzed. In all six cases, central chromatolytic changes in the ventral horn cells were commonly observed. In addition, two out of four GBS cases and one of two CIDP cases showed a significant reduction in the ventral horn cell population, particularly in neurons with a large diameter. The cases with significant motoneuron loss also showed a remarkable reduction in the number of myelinated fibers in the ventral spinal roots, and severe axonal degeneration was observed in cases in acute phase. Astrogliosis of the ventral horn was also seen in some cases. The present study suggested that motoneuron loss in the primary demyelinating diseases like GBS and CIDP is the consequence of the axonal pathology of the motoneuron axons. These observations are helpful to understand motoneuron loss in certain motoneuron diseases with preferential involvement of lower motor neurons.