Little is known about the optimal management of patients with chronic hepatitis B (CHB) who develop drug resistance. The aim of this study was to investigate the effectiveness of different drug regimens in chronically HBV-infected patients. HBV viral load was determined using a bDNA assay and the substitutions in HBV-DNA were studied by polymerase sequencing test. The study involved 38 patients who experienced a therapeutic failure to lamivudine (LAM). The sequential treatments used were: LAM + adefovir (ADV), LAM + tenofovir (TDF), entecavir (ETV) monotherapy, ADV monotherapy and TDF monotherapy. Similar activity against HBV replication was observed with all drug regimens. Of the patients treated with LAM, 44% developed resistance mutations. The rt M204I mutation was observed more frequently. Sequential ADV add-on LAM and TDF therapy induced the appearance of resistance in 3/18 (16.6%) and in 1/8 (5.5%) treated patients, respectively. Genotype D was the most prevalent (78.9%), followed by genotype A (13%), genotype E (5.2%) and genotype C (2.6%). Our study showed that baseline serum HBV DNA is an important predictor of virologic response and that virologic breakthrough is significantly associated with the insurgence of genotypic resistance.