Two successive protocols of phenytoin (PHT) plasma concentration monitoring were tested in 60 children with status epilepticus (SE). In each protocol, a loading dose of 15 mg/kg was injected and followed by three injections during the first 24 h. Clinical evaluation was performed at the end of the study by grouping patients into three classes according to seizure frequency during treatment: complete effect, partial effect, and no effect. In protocol 1, a complementary dose at the fourth hour was adjusted from individual plasma concentrations. Plasma concentrations at the 40th hour were within the therapeutic range in the 19 patients with complete effect (CE, mean 19 mg/L) and in the 5 patients with no effect (NE, mean 23 mg/L) whereas in the 11 patients with partial effect (PE), plasma concentrations were higher (mean 31 mg/L). In protocol 2, we added monitoring of the doses injected at the 16th and 24th hours to prevent the increase in PHT concentration noted in protocol 1. All patients but 1 were classified as either CE (13 patients) or NE (4 patients). In NE patients, average plasma concentration at the 40th hour (mean 10.5 mg/L) was lower than in CE patients (mean 15.7 mg/L). In both protocols, the NE patients were the youngest. In SE, PHT doses should be adjusted according to plasma concentrations to avoid overdosage and paradoxical inefficacy. Younger children had lower concentrations and appeared to respond less well than older children, but the etiology of SE may also play an important role.