Atypical clinical and/or EEG presentation may complicate the diagnosis of juvenile myoclonic epilepsy (JME). To assess the sensitivity of a standard EEG recording, we retrospectively evaluated the EEG performed at their first referral in 56 consecutive JME patients first seen between 1986 and 1992 (26 M, 30 F, aged 12-53, mean 24.4, with onset of JME at age 10-33, mean 14.3). The diagnosis had been made in none of these patients prior to referral, and was often confirmed only during follow-up. A 20-minute standard EEG was recorded, including hyperventilation (HV) and intermittent light stimulation (ILS). This EEG was normal in 15 cases (27%), showing aspecific or misleading changes in 11 cases (20%) and typical changes in only 30 cases (54%). The baseline EEG was normal in 25 (45%), atypical in 11 (20%), and typical for JME in only 20 (35%). HV and ILS yielded 37 and 39 normal, 10 and 7 aspecific and 9 and 10 specific findings, respectively. A single standard EEG without activation may thus be inconclusive or misleading for the diagnosis of JME in more than 50% of newly referred patients.