The right and left biceps and first dorsal interosseous muscles of 22 patients with motor neurone disease were studied by single fibre EMG at the time of diagnosis. The mean duration of symptoms was 8.1 months. The fibre density was increased in 87 of the 88 muscles studied. In the first dorsal interosseous muscles the fibre density was increased similarly on both sides, but in the biceps muscles of normal strength the fibre density was more markedly increased on the left than on the right. These findings suggest that in the early stage of the disease certain motor unit pools in the spinal cord are preferentially affected.