We reviewed all cases of infectious esophagitis diagnosed after marrow transplantation during a 2-yr period. Twenty-one infections were found during 46 fiberoptic endoscopic examinations; most infections were viral, with equal numbers due to cytomegalovirus and herpes simplex virus. These infections presented with nonspecific esophageal symptoms weeks to months after transplantation. Diagnoses depended on histology and viral cultures. During the same time span, autopsies revealed 25 cases of infective esophagitis among 59 patients; most were again viral, with equal numbers of herpes simplex virus and cytomegalovirus infections. Mixed infections due to viral, fungal, and bacterial pathogens were found in both endoscopy and autopsy series. The prevalence of esophageal infection in marrow transplant patients with esophageal symptoms is high, but the organisms could not be predicted on the basis of symptoms, oropharyngeal surveillance cultures, or esophageal x-ray. Accurate diagnosis requires that endoscopic biopsy specimens and brushings be examined by both histologic and virologic methods.