There are remarkable advances in the treatment of chronic hepatitis B (CHB) in the last few years. Unfortunately, prolonged antiviral treatment is associated with increasing risk of drug resistance/viral breakthrough (VBT), which may lead to flare-up and rapid decompensation. We have designed this study to predict the pretreatment and on-treatment factors responsible for development of VBT.This study was conducted during the period of February 2000 to November 2007. We have included 423 patients who received lamivudine (LAM) therapy for at least 1 year and at least 2 follow-ups at 6 months' interval. Follow-up period was 12-78 months. Chi-square test, student's t test, and logistic regression analysis were performed to prove the validity.Of the 423 study cases, 367 (86.8%) were of male patients and 261 (61.7%) patients were HBeAg positive; the age of the patients was 30.8 +/- 12.9 years. Development of VBT was 4.4, 22.8, 45.3, and 74% at 1, 2, 3, and 4 or more years, respectively. Pretreatment high HBV DNA (P = 0.005) and female sex (P = 0.01) were associated with VBT and pretherapy ALT (P = 0.698), HBeAg status (P = 0.273), and age (P = 0.059) were not associated. Duration of treatment, failure to lose HBeAg at 1 year, and HBV DNA nonresponder at 6 months were significantly (P = 0.001) associated with development of VBT.Persistence of HBeAg at 1 year and HBV DNA nonresponder at 6 months are good predictors of development of VBT.