We describe a case of a 9-year-old boy with the severe acute liver injury caused by idiosyncrasy after the administration of valproate. The liver biopsy performed during the first days after the onset of the disease revealed the necrosis of more than 50% of hepatocytes but the injury was almost fully reversible which was well demonstrated by the second biopsy five months later. After five years an evidence for the immunologic idiosyncrasy caused by valproate was still present, which was demonstrated in vitro by blastic transformation of lymphocytes from the patient. The boy is quite healthy and epilepsy is under control of suximide. The family was informed that the boy could be treated with valproate never more.