Herpes simplex virus (HSV) infection in adult patients who underwent cord blood transplantation (CBT) from unrelated donors was studied. None of nine HSV-seronegative patients developed HSV disease after CBT. Of 28 HSV-seropositive patients, seven (25%) developed HSV disease at a median of 92 days after CBT (range, 52-239 days). The cumulative incidence of HSV disease in HSV-seropositive patients was 27% at 12 months after CBT. The manifestations of HSV disease included gingivostomatitis (three patients), herpes labialis (two patients), localized herpes facialis of the nose (one patient), and disseminated eczema herpeticum (one patient). HSV disease recurred in two patients as gingivostomatitis and disseminated eczema herpeticum. All the patients responded to antiviral therapy. The presence of grade II-IV acute graft-versus-host disease (GVHD) was significantly associated with a higher rate of HSV disease after CBT (51 vs 8%, P=0.015). These results suggest that the recovery of HSV-specific immune responses is delayed in patients who develop grade II-IV acute GVHD after CBT.