We herein report on an 18-year-old Taiwanese man who developed acute onset of generalized continuous contractions of the muscles. The involuntary cramps caused a dystonic posture of the neck and limbs. Opisthotonos was noted when the cramps were exacerbated. The cramps were painful and sensitive to movement, so the patient limited movement of his body. External stimulation, such as a pin prick or a loud noise, would aggravate them. The contractions of the muscles persisted during sleep. No other neurologic abnormalities were noted. Electromyography showed spontaneous continuous muscle fiber activity. The symptoms did not respond to baclofen or diazepam; however, remarkable improvement was noted after giving phenytoin. The clinical presentation, electromyographic findings, and the patient's beneficial response to phenytoin fulfilled the diagnostic characteristics of Isaacs syndrome.