While treatment of herpes simplex and varicella-zoster virus infections, such as respiratory syncytial, influenza, and human immune deficiency viruses, has dominated clinical antiviral trials, recent reports of ganciclovir, also called 9-(1,3-dihydroxy-2-proxymethyl) guanine (DHPG), as a possible therapy for cytomegalovirus infection in high-risk, immunocompromised persons, have attracted increasing attention. A new drug application is currently under review by the Food and Drug Administration for ganciclovir therapy for cytomegalovirus chorioretinitis in immunocompromised patients (14-15). These new studies introduce two major considerations: host-virus interaction and the design of clinical trials to evaluate new therapies for high-risk patients.