Hepatitis B vaccination and targeted testing for hepatitis C virus (HCV) are recommended for jails with medical services available. This study estimates hepatitis B virus (HBV) and HCV infection prevalence among jail inmates, since most previous studies have been conducted among prison inmates. Prison and jail populations differ: jails hold a wide spectrum of persons for an average of 10-20 days, including persons awaiting arraignment, trial, conviction, or sentencing, while prisons typically hold convicted criminals for at least 1 year. A stratified random sample of sera obtained during routine syphilis testing of inmates entering jails in Chicago (March-April 2000), Detroit (March-August 1999), and San Francisco (June 1999-December 2000) was tested for serologic markers of HBV and HCV infection. All sera had been previously tested for antibody to HIV (anti-HIV). A total of 1,292 serum samples (12% of new inmates) was tested. Antibody to HCV (anti-HCV) prevalence was 13%. Antibody to hepatitis B core antigen (anti-HBc) prevalence was 19%, and hepatitis B surface antigen (HBsAg) prevalence was 0.9%; 12% had serologic evidence of hepatitis B vaccination. Hispanics had high rates of chronic HBV infection (3.6% HBsAg positive) along with Asians (4.7% HBsAg positive). Among HIV-infected persons, 38% were anti-HCV positive and 8.2% were HBsAg positive. Anti-HBc positivity was associated with anti-HCV positivity (aOR=4.58), anti-HIV positivity (aOR=2.94), syphilis infection (aOR=2.10), and previous incarceration (aOR=1.78). Anti-HCV-positivity was associated with anti-HBc positivity (aOR=4.44), anti-HIV-positivity (aOR=2.51), and previous incarceration (aOR=2.90). Jail entrants had high levels of HCV and HBV infection and HIV co-infection; HBV prevalence was comparable to previous prison studies, and HCV prevalence was lower than prison studies. Hispanics had an unexpectedly high rate of chronic hepatitis B infection and had the lowest rate of hepatitis B vaccination. The finding that hepatitis B vaccination coverage among jail entrants is lower than the general population, despite this population's increased risk for infection, highlights the need to support vaccination in jail settings.