In many Asian countries the ratio of male to female population is higher than in the West -- as high as 1.07 in China and India, and even higher in Pakistan. A number of authors (most notably Sen, 1992) have suggested that this imbalance reflects excess female mortality and, as a result, have argued that as many as 100 million women are "missing." This paper proposes an explanation for much of the observed over-representation of males: the hepatitis B virus. Evidence drawn from the existing medical literature as well as new studies of recent vaccination efforts indicate that carriers of the hepatitis B virus have offspring sex ratios as high as 1.55 boys for each girl. This is strongly supported by cross-country evidence on hepatitis B prevalence and sex ratios at birth. Hepatitis B is common in many Asian countries, especially China, where some 10 to 15% of the population is infected. Using data on viral prevalence by country as well as estimates of the effect of hepatitis on sex ratio drawn from a wide range of sources, I find that hepatitis B can explain about 45% of the missing women: around 75% in China, between 20% and 50% in Bangladesh, Egypt, and West Asia, and under 20% in India, Pakistan and Nepal.