This study aimed to clarify factors associated with intravenous administered phenytoin-induced hypersensitivity reaction. The incidence of hypersensitivity was significantly more frequent in boys than in girls (P < 0.05). Patients with hypersensitivity were relatively younger than those without hypersensitivity, although the difference was not statistically significant. There was no relation between the initial dose or maximum blood level of phenytoin and the occurrence of hypersensitivity. The initial serum level of phenytoin was significantly lower in patients with hypersensitivity than in those without hypersensitivity (P < 0.05), whereas the total dose of phenytoin was relatively larger in patients with hypersensitivity than those without. Reactivation of human herpes virus-6 was not recognized in all 3 patients in whom virological examination was performed using real-time polymerase chain reaction.