A case of phenytoin-induced hepatitis with mononucleosis is reported, and syndromes associated with phenytoin hypersensitivity reactions are discussed. A 23-year-old black woman with a two-month history of seizure disorder was admitted to a hospital with nausea, vomiting, fever, lymphadenopathy, diffuse maculopapular rash, left-upper-quadrant tenderness, and hepatomegaly. She was receiving phenytoin sodium 300 mg/day; carbamazepine 200 mg four times daily had been discontinued four days before admission because of leukopenia. Phenytoin was discontinued after admission; however, phenytoin 1 g i.v. was given for a tonic-clonic seizure two days after admission, after which swelling of the face and legs and pruritus developed. Over the next few days, signs and symptoms of hepatotoxicity progressed, and she became comatose. Seizures were treated with diazepam. She began to recover after 10 days of supportive therapy and was discharged several weeks later on primidone therapy. Serious phenytoin hypersensitivity reactions may appear as dermatologic, lymphoid, or hepatic syndromes. Fever, rash, and lymphadenopathy often accompany hepatic injury. Encephalopathy and death may occur. Proposed mechanisms for phenytoin hypersensitivity include antigen-antibody reactions, alteration of lymphocyte function, and an enzyme abnormality causing the production of toxic metabolites. Treatment is supportive; phenobarbital and carbamazepine may be used with caution as alternate anticonvulsant therapy. The possibility of phenytoin hypersensitivity reactions should be considered when patients receiving phenytoin have unusual symptoms, particularly fever, rash, and lymphadenopathy.