A 23-year-old female was admitted to our hospital because of unilateral calf enlargement. On neurological examination, she was mentally retarded with mild dysarthria. Grip myotonia was bilaterally present. Muscle weakness including those of face, neck and extremities was noted. The right calf tight and firm on palpation, was markedly enlarged with a circumference of 35.0 cm compared with 30.0 cm on the other side. Computed tomographic scans of the skeletal muscles revealed marked hypertrophy in the right triceps surae muscle. Electromyographic studies demonstrated persistent myotonic discharge in the hypertrophied gastrocnemius muscle. A muscle biopsy specimen obtained from the right gastrocnemius showed many hypertrophied muscle fibers, with prominent internal and sarcolemmal nuclei. Histographic analysis indicated a hypertrophy factor of 2,025 for type 1 fibers and 1,102 for type 2 fibers. DNA analysis by Southern hybridization technique showed large DNA fragment, consistent with congenital myotonic dystrophy. In the present patient, unilateral calf enlargement was an unusual feature associated with myotonic dystrophy. Neuroradiological and pathological studies confirmed true muscle hypertrophy of unilateral calf muscle. A search of the literature failed to reveal any case similar to our patient.