A 46-year-old male patient was irradiated following left orchiectomy for seminoma. Three months after radiation, segmental myoclonus appeared involving all skeletal muscle groups up to the level of D10, the area of radiation. Urodynamic testing showed the presence of segmental myoclonus in the urethral sphincter. An around-the-clock ‘continuous segmental myoclonus’ during sleep and wakefulness with a frequency of 1 c/s and an amplitude of 400 µV selectively inhibited by REM sleep was not found in the literature. This continuous segmental myoclonus caused by a radiation myelopathy at D10-D11 was followed up for 2 years. It was partially controlled by carbama-zepine.