Hepatitis B virus (HBV) reactivation is a well-known complication after rituximab therapy in patients with B cell lymphoma. Traditionally, hepatitis B surface antibody (anti-HBs) is a protective antibody, but the effect of rituximab on these antibodies has not been well studied. In 29 B cell lymphoma patients who were positive for anti-HBs before rituximab therapy, anti-HBs serologies before and after rituximab therapy were compared. Anti-HBs titers after rituximab treatment were significantly lower (P < 0.001) than those before treatment. None of the ten cases with pre-treatment anti-HBs titers above 100 mIU/mL became negative for anti-HBs after rituximab therapy. In contrast, 8 of the 19 patients with pre-treatment anti-HBs titers below 100 mIU/mL lost their anti-HBs (P = 0.027). Of these, one patient developed HBsAg seroreversion and HBV reactivation after rituximab therapy. Regarding patients with loss of anti-HBs or not, there was no significant difference in pre- and post-treatment immunoglobulin G levels between both groups. The rate of anti-HBs loss increased with advanced lymphoma stage and international prognostic index (P = 0.002 and <0.001, respectively). Multiple logistic regression analysis showed that pre-treatment anti-HBs titer is the only independent factor influencing the loss of anti-HBs (per one log mIU/mL, odds ratio, 0.003; 95% confidence interval, 0.000-0.302; P = 0.014). In conclusion, we found that anti-HBs titers decreased significantly (P < 0.001) after rituximab treatment. B cell lymphoma patients with low pre-treatment anti-HBs titers (<100 mIU/mL) were more likely to lose anti-HBs antibodies and were at risk of HBV reactivation after rituximab immunochemotherapy.