Bromovinyldeoxyuridine (BVDU) is a highly potent and selective antiherpetic agent which offers great potential for the treatment of severe herpes simplex virus type 1 (HSV-1) and varicella-zoster virus (VZV) infections in cancer patients. BVDU inhibits the replication of HSV-1 and VZV at a concentration as low as 1-10 ng/ml; and the proliferation of tumor cells transformed with the HSV-1 thymidine kinase gene is even inhibited by BVDU concentrations lower than 1 ng/ml. Moreover, BVDU is inhibitory to Epstein-Barr virus replication in vitro at a concentration of 0.02 micrograms/ml. Due to the action of pyrimidine nucleoside phosphorylases, BVDU is rapidly degraded to the free pyrimidine base bromovinyluracil (BVU). In contrast to BVDU, which is cleared from the bloodstream within 2-3 hours, BVU persists in the plasma for at least 24 hours. During this period BVU can be converted again to BVDU upon administration of deoxythymidine, deoxyuridine or any other deoxyribonucleoside capable of transferring its deoxyribosyl moiety onto BVU. BVU owes its long persistence in the bloodstream to the fact that it does not act as substrate for dihydrothymine dehydrogenase, the enzyme that catalyzes the first step in the catabolic pathway of pyrimidines. On the contrary, BVU acts as an efficient inhibitor of this enzyme and thereby prevents the degradation of fluorouracil (FU), a well-known anticancer agent. As a consequence, BVDU via BVU enhances the antitumor activity of FU, as has been demonstrated in the murine P388 leukemia model. Thus, BVDU may be useful in anticancer chemotherapy from several viewpoints, e.g. for treatment of intercurrent herpesvirus infections, and, in combination with FU, for treatment of those malignant diseases that are amenable to FU therapy.