Single fibre EMG was used to study the motor unit fibre density in the right biceps brachii, extensor digitorum communis and first dorsal interosseous muscles of 15 patients with motor neurone disease, with different patterns of initial weakness. There was an inverse relationship between strength and fibre density in these muscles. Abnormalities were more marked in patients whose initial symptom was arm weakness, but collateral reinnervation was not as effective as in other neurogenic disorders. These findings are consitent with a hypothesis that motor neurone disease begins segmentally, or at a discrete level within the motor system, before becoming generalised. The single fibre EMG fibre density is a useful quantitative technique for sequential assessment of patients with neurogenic disorders.