The case of a 15-year-old male patient, suffering from difficulty in moving his arms and opening his mouth and dyspnea which had been slowly developing for two years, is reported. The neurologic examination revealed diffuse hypertonus, predominating in some muscle groups, compelling the patient to assume abnormal attitudes and making it impossible for him to open his mouth and to breathe freely due to a large decrease in the thoracic expansibility. The electromyographic studies showed continuous muscle activity at rest, characterized by high frequency discharges, which continued after anesthetic blocking of a peripheral nerve and after general anesthesia; these discharges were eliminated only when curare was used. Diagnosis was made of scleroderma and myositis, although only the first one could be confirmed by biopsy; the patient was treated with corticosteroids and a slight improvement was evidenced, specially in regard to breathing movements and mouth opening. Based on the good results reported by Isaacs in two similar cases, the authors used sodium hidantoinate, and thus found a marked decrease of the abnormal electric activity, proved though an electromyographic examination made some days afterwards.