We studied myoclonic seizures (MS) in 5 patients with juvenile myoclonic epilepsy (JME) using video polygraphic recordings to investigate the clinical characteristics of MS in this epileptic syndrome. The total number of MS analyzed was 302 (range 27-125, mean 60) seizures per patient. MS occurred either singly or repetitively (37 vs. 63%) and corresponded to generalized bilaterally synchronous single or multispike-and-wave complexes at 3-5 Hz. Video analysis of the myoclonic jerks demonstrated that either distal or proximal muscle involvement predominated. In the former, there was mild bilateral flexion and some external rotation of the forearms. In the latter, flexion of both arms at the elbow, flexion and abduction of the thighs, and extension of the back was observed. Asymmetry of MS was noted in 4 of 5 patients. Facial involvement of MS occurred infrequently in 2 patients. When the patients kept both arms outstretched, the arms dropped or there was sudden interruption of ongoing electromyographic (EMG) potentials immediately after myoclonic jerks (postmyoclonic inhibition) in all patients. One should inquire about these clinical characteristics of MS in JME when taking a thorough history in patients with primary generalized tonic-clonic seizures (GTC).