We examined 26 spinal muscular atrophy type III (SMA III) patients with SMNt deletions, searching for possible segmental distribution of muscle weakness. in those with disease duration of less than or equal to 11 years, the weakest muscles were upper lumbar innervated ones in the lower extremities. In the upper extremities, early involvement of triceps muscle suggested the possibility of lower cervical (C7) onset. Electrophysiologically, weaker muscles had a more severe reduction in the recruitment pattern, particularly in the lower extremities. However, severe reduction in recruitment was sometimes also observed in clinically strong muscles, In patients with disease duration of greater than or equal to 16 years and regardless of disease duration, in those with disease onset at less than or equal to 3 years of age, weakness and severe electrophysiological changes were more widespread. These findings may suggest a progression in muscle weakness with time. When compared to 12 patients with Becker muscular dystrophy (BMD), early stage SMA III with weak iliopsoas-strong gluteus maximus stood in contrast to BMD with weak gluteus maximus strong iliopsoas. (C) 1997 Elsevier Science B.V.