Serum levels of mephenytoin (Mesantoin) and its metabolite nirvanol were correlated with effectiveness and side effects in 93 patients. Mean mephenytoin level was 8% of the combined mephenytoin plus nirvanol levels. "Total mephenytoin" level should be used clinically, as neither individual component is as well correlated with clinical phenomena. Serum levels of 25 to 40 mug/ml usually yield improvement in seizure control without discomfort, and three-quarters of patients had fewer seizures. Side effects frequently associated with phenytoin were absent, but drowsiness, an occasional rash, and a single, fatal case of aplastic anemia were found. Performance on psychological tests of cognitive-attentional skills showed a modest improvement during mephenytoin administration. The drug merits wider employment in refractory seizure problems, but vigilant follow-up is required.