The polymerase chain reaction (PCR) was used to search for hepatitis B virus (HBV) DNA sequences in the sera of 51 dialyzed patients (26 women, 25 men; mean age 60.5 years, range 35 to 85). Two different sets of specific primers for HBV core and surface gene sequences were synthesized and used for each sample. Controls were 90 HBV negative blood donors. Results were analyzed according to other serological markers of HBV. Among the eight HBsAg positive patients (anti-HBc+: 8/8), seven were positive for HBV DNA. Four of eight patients were vaccinated but later developed acute HBs hepatitis. The presence of HBV DNA was detected in six of 43 HBsAg negative patients (anti-HBc+: 5/6; anti-HBs+: 3 of 6; HBeAg: 0 of 6; anti-HBe: 2 of 6). These six patients were vaccinated and four of six developed mild and transient cytolytic hepatitis (3 before vaccination; 2 later). These results showed that HBsAg seronegative patients can be infectious. The role of HBV vaccination and/or the existence of variations in the structure of the viral genome is discussed.