The EEG features and clinical correlates were investigated before, directly after, and on long-term follow-up after initiation of pyridoxine therapy in 6 patients with B6-dependent epilepsy. At each phase, the EEG provided important diagnostic and prognostic information. Pre-B6 3 neonates manifested a unique EEG pattern of generalized bursts of 1-4 Hz sharp and slow activity. This pattern has not been previously described in neonates with B6 dependency and in this age group appears to be highly suggestive of the diagnosis. Five patients experienced an apparent initial response to traditional antiepileptics. The parenteral pyridoxine test, performed in all 5, and repeated in 3, proved to be a highly reliable and reproducible diagnostic test. After 50-100 mg of B6 there was cessation of clinical seizures within minutes and of paroxysmal discharges within hours. On long-term follow-up (3-28 years) all 6 patients were seizure free on B6 (10-100 mg/day) monotherapy. Recurrences of seizures and of specific sequential EEG changes (background slowing, photoparoxysmal response, spontaneous discharges, stimulus-induced myoclonus, generalized seizures) occurred upon B6 withdrawal. Long-term prognosis correlated with the EEG. Two patients had persistently abnormal EEG backgrounds and were moderately to severely retarded, while 4 had normal EEGs with normal or near normal development.