The era of pharmacoepidemiology has introduced profound changes in the ability of the sector to monitor the post-marketing safety of new products. We report on the population-based assessment by Wellcome of the safety of Zovirax reflected in a program of unprecedented scope, diversity, and, most important, utility. The program couples epidemiologic intelligence--analysis of adverse experience reports arising spontaneously from practice and reported directly to the manufacturer, regulators, or in the published literature--with more structured formal epidemiologic research approaches. Taken together, the absence of major medical problems emerging from the monitoring system for spontaneous reports from the medical practice experience among over 20 million persons treated worldwide over the past decade, complemented by the similar absence of "signals" from structured epidemiologic studies closely monitoring the experience of over 50,000 patients, constitutes a persuasive body of information concerning the general safety of this important therapeutic intervention.