Forty-seven spinal cords from subjects between 13 and 95 years of age have been examined; in the majority death occurred suddenly or rapidly from a state of previous good health. The limb motor neurons in the lumbospinal segments have been estimated by counting nucleoli in characeteristic cells in every fifth group of 10 serial sections from the entire lumbosacral cord. No evidence exists of loss of motor neurons up to the age of 60 years, but beyond that age, although individual counts vary considerably, there is increasing evidence of a diminishing motor neuron pupulation; above 60 years, several cases showed motor neuron counts of only 50% of the counts in early adult life or middle age. Cell loss appeared to be uniform throughout all the segments and was unaccompanied by any other striking morphological change.