We studied 21 patients with complex partial seizures during phenobarbital (PB) or primidone withdrawal. Blood levels were measured daily, and seizure frequency was monitored by nursing staff and EEG-video telemetry. Patients were monitored for one week of baseline and for five weeks after PB tapering was initiated (withdrawal). Most were observed for at least one week after levels were undetectable. When ranked seizure counts were averaged, there was a tendency for seizure rates to be highest as PB levels passed through the range 15 to 20 mg/l, compared with rates at higher or lower PB levels. There was little or no relation between the rate of PB withdrawal, other drug therapy, or initial PB level and the ratio of highest seizure frequency to mean seizure frequency. Patients withdrawing from PB may be most likely to experience an increase in seizure frequency when their PB blood level falls below 20 mg/l.