To study the cognitive effects of diphenylhydantoin (PTH), 17 patients of simple partial or generalised tonic clonic seizure, who were on PTH monotherapy for at least 6 months were included. None of them had a seizure in last one month and their CT scans were normal. Serum PTH levels of these patients were in the therapeutic range (10.2-17.7 micrograms/ml). The clinical signs of PTH toxicity in the form of gingival hyperplasia and cerebellar ataxia were present in 1 patient only. In 3 patients memory quotient (MQ) and Benton visual retention test (BVRT) scores were marginally abnormal but the group difference in the clinical psychometric tests were not significant. P3 latency was also normal in all the patients. In the therapeutic range, PTH does not seem to produce significant cognitive dysfunction.