The hydantoin derivatives and analogues may, in sensitive patients, induce a syndrome which so closely mimics lymphosarcoma, both clinically and pathologically, that it leads to long and costly hospitalization. A case is reported of a patient under treatment with diphenylhydantoin sodium for grand mal epilepsy. She was admitted to hospital with fever, joint swellings, facial edema, generalized aches and pains, and adenopathy. A lymph node biopsy led to the diagnosis of lymphoblastic lymphosarcoma, and the patient was treated with nitrogen mustard. A continuing adverse clinical picture eventually prompted withdrawal of the diphenylhydantoin sodium. Gradual clinical improvement followed. Resumption of the drug by the patient after discharge from the hospital caused a recrudescence, and she was readmitted. After withdrawal of the diphenylhydantoin sodium, the symptoms again subsided.