In a longitudinal investigation of 14 patients with motor neuron disease motor units were studied in the biceps and first dorsal interosseous muscles in both arms, using single fibre EMG. The fibre density usually increased initially and this was occasionally accompanied by temporary improvement in strength. Later, as the fibre density decreased, increasing weakness and atrophy developed. The fibre density was rarely as high at any stage of the disease as in other chronic neurogenic disorders. Marked asymmetry was observed in individual patients at all stages of the disease.