Acquired neuromyotonia (Isaacs' syndrome) is a rare disorder characterized by hyperexcitability of peripheral motor nerves. The cardinal features consist of myokymia, pseudomyotonia and contracture of hands and feet. The diagnosis of Isaacs' syndrome is based on the clinical features and classic electromyographic findings. Serum antibodies against Voltage-Gated Potassium Channels (VGKCs) are detected in some cases. The authors report a 17 year-old man presented with difficulty in walking, writing and respiratory discomfort for 7 months. His body weight had decreased from 120 to 70 kilograms during that period. Physical examination was remarkable for profound sweating. Muscles were in a state of contraction, action myotonia without percussion myotonia, myokymia and carpopedal spasm. Electromyography showed classical neuromyotonic and myokymic discharges. The investigations for conditions associated with Isaacs' syndrome were unrevealing. VGKCs antibody were not performed. Treatment with carbamazepine resulted in substantial improvement of the symptoms within 7 days.