Myotonic dystrophy is a neuromuscular disorder characterized by muscle weakness and myotonia. Myotonia manifest as abnormally slow relaxation after strong voluntary contraction. So patients with myotonic dystrophy are difficult to perform activities of daily living. The purpose of this study was to assess the degree of muscle weakness and myotonia in 10 patients with myotonic dystrophy, and to determine the effect of a four-week therapeutic trial of quinine sulfate quantitatively. The patients didn't recieved any rehabilitative treatment including muscle strengthening exercise during medication. Muscle strength was quantified by comparing threesecond maximum voluntary contraction of 10 patients with that of 20 healthy subjects. First dorsal interosseous, elbow flexor, and knee extensor were compared. Myotonia was quantified by measuring relaxation times at the end of three-second maximum voluntary contraction produced by First dorsal interosseous, elbow flexor, and knee extensor as the time taken from the maximum voluntary contraction to decrease by 100%. The change of amplitude of the evoked muscle action potentials between before and after ten-second maximum voluntary contraction were assessed from the first dorsal interosseous and abductor digit quinti muscles. The results were as follows:(1) The mean muscle strength of each of the three muscles of the patients was significantly reduced compared with healthy subjects; and (2) Relaxation times of the three muscles of the patients were significantly prolonged compared with healthy subjects. Seven of the ten patients participated in a therapeutic trial of quinine sulfate. Therapeutic effects were also assessed by measuring muscle strength, and relaxation time. Relaxation times were reduced significantly in all three muscles after medicaton with quinine sulfate. However, there were no significant improvement of muscle strength of all the three muscles. We concluded that quinine sulfate may provide therapeutic benifit to myotonia but not muscle weakness. So for improvement of muscle strength, comprehensive rehabilitative treatment based on muscle strengthening exercise should be combined.