Herpes viruses have repeatedly been associated with the etiology of multiple sclerosis (MS). In the course of investigations on a cluster of MS patients in Key West, Florida, it was suggested that Marek's disease virus (MDV), a cell-associated avian herpes virus, might be involved in the pathogenesis of the disease. The aim of our study was to investigate 107 well-defined MS patients with regard to latent MDV infection. Using a highly- sensitive polymerase chain reaction technique, we did not find MDV-related sequences in leukocyte DNA of any of the patients. The results of our study do not suggest an implication of MDV in the pathogenesis of MS.