The ultrastructure of human intramuscular nerves at biopsy has been compared in motor neurone disease with that in other neuromuscular disease including muscular dystrophy and in controls. Myelinated axons appeared to be lost in control patients aged over 50 years, but this varied both between and within nerve fascicles. Even in two boys aged 31/2 and 9 years, but with Duchenne's dystrophy, examples were found of nerve fascicles with few or no axons. In motor neurone disease additional nerve fibres were lost, but there was little change in the size distributions of axons and myelin sheaths within the muscle. In both preterminal and terminal fascicles there was an increase of Schwann cell cytoplasm in association with unmyelinated axons, which was compatible with nerve sprouting. It is concluded that intramuscular nerves are likely to be lost with age as well as motor neurone disease. The results are discussed in relation to the stage of the disease process at biopsy.