We report a case of phenytoin toxicity due to impaired drug metabolism in a patient homozygous for $CYP2C9^{ast}3$. A 46-year-old woman was taking phenytoin to prevent postoperative seizures. She attained high serum phenytoin levels at the standard doses (300 mg/day) and developed symptoms of phenytoin toxicity including blurred vision, nausea and headache. The patient was treated with reduced doses of phenytoin and then phenytoin therapy was finally discontinued. Genotyping for CYP2C9 revealed that this patient had a homozygous genotype, $CYP2C9^{ast}3/^{ast}3$. This is the first Korean case of phenytoin toxicity with homozygous $CYP2C9^{ast}3$. This case suggests the clinical usefulness of pharmacogenetic testing for individualized dosage adjustments of phenytoin.