Multivariate analysis of the risk factors for hepatitis D virus infection among 53 men with acute hepatitis D virus superinfection, 59 men with acute exacerbation of chronic hepatitis B and 54 asymptomatic male hepatitis B carriers revealed that sexual contact with prostitutes and use of nondisposable needles were the factors significantly associated with hepatitis D virus infection in Taiwan (odds ratio = 5.5, 95% confidence interval = 3.3 to 9.2; odds ratio = 2.4, 95% confidence interval = 1.3 to 4.4, respectively). The prevalence of antibody to hepatitis D virus antigen among hepatitis B virus carriers with histories of sexual exposure to prostitutes was 10.3% (14 of 136) and the prevalence of hepatitis D virus antigen among the female prostitutes who were hepatitis B virus carriers was 59% (24 of 51). Among those who admitted to sexual contact with prostitutes, testing for hepatitis D virus antigen revealed positive correlations with the frequency of such sexual contacts and with the frequency of history of venereal diseases. Among the female prostitutes, the prevalence of hepatitis D virus antigen had a positive correlation with the frequency of sexual contact and a negative correlation with age. Analysis of serum hepatitis D virus RNA among those with hepatitis D virus antigen revealed that 4 of the 9 with sexual exposure to prostitutes and 3 of the 24 prostitutes were positive. From these findings, we conclude that these two groups are not only at high risk for hepatitis D virus infection but also form a reservoir of this virus.(HEPATOLOGY 1990;11:1057-1061.).