Evaluation of resistance pattern in patients with chronic hepatitis B. Retrospective study of hepatitis B virus (HBV) resistance mutations in patients found viraemic after first-line treatment. HBV viral load was determined by a real-time polymerase chain reaction and the substitutions in HBV-DNA were studied by polymerase sequencing test. First line treatment had failed in 12 out of 33 patients (36%) receiving anti-HBV drugs. The 12 patients with persistent viraemia were all lamivudine (LAM) experienced and 7 had a polymerase sequencing test available. LAM substitution mutations L180M + M204V/I were found in six out of seven cases, with an accompanying V173L mutation in three cases. These mutations were also related with changes in HBsAg. The use of potent drugs in the first line anti-HBV therapy may reduce the resistance mutations in the future. J. Med. Virol. 83:1905–1908, 2011. © 2011 Wiley-Liss, Inc.