Interstitial pneumonia caused by Herpes simplex virus type 1 (HSV-1) is a severe complication of orthotopic liver transplantation (LTX). The records of patients were reviewed who had an LTX at the age of 16 years or older between 1991 and 1994 with a mean follow-up of 21 months (range, 10 to 44 months). Six patients were included who had fever of > 38 degrees C, deterioration of arterial blood gases, radiological evidence of interstitial pneumonia and proof of HSV-1 in bronchoalveolar lavage fluid. All patients were anti-HSV-IgG positive before LTX. All patients were successfully treated with intravenous acyclovir, mechanical ventilation and reduced immunosuppression. Three patients who received cyclosporin A had a rejection which was successfully treated by switching to FK 506. Four patients were discharged in good health. One patient died 36 months after LTX of an unrelated cause. One patient died of urosepsis on postoperative day 139. Acyclovir together with mechanical ventilation and reduced immunosuppression proved to be an effective treatment for HSV-1 pneumonia following LTX.