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.