The aim of the present study was to examine whether there is an abnormal expression of certain endogenous retroviruses in MS patients. For this purpose samples of peripheral blood mononuclear cells were obtained from 22 MS patients, a corresponding number of age and sex-matched healthy donors and five patients with other diseases affecting the central nervous system. In addition, brain specimens of macroscopic normal white and gray matter from four MS patients and a similar number of controls were included in the study. Using an enzymatic amplification technique, we found expression of the endogenous retroviral sequences, HRES-1, HERV-K10 and ERV3 in most samples of peripheral blood mononuclear cells from MS patients and controls without obvious differences between these two groups. In contrast, composite transcripts of ERV3 and a zinc finger sequence were more frequently detected in healthy donors than in MS patients. At present, the possible significance of this is uncertain. The retroviral element 4-1 was not transcribed or only transcribed at a very low level in peripheral blood cells of controls and MS patients. Transcripts of various endogenous retroviruses were also detected in the brain samples, but a different pattern was not apparent in the MS group as compared with controls. Aspects concerning a possible association between endogenous retroviruses and autoimmunity are considered.