Background: Reactivation of HBV has often been reported in patients (pts) with malignant lymphoma administered combination chemotherapy regimens containing rituximab. Recently, HBV reactivation has been reported even in pts with STs. However, the incidence of HBV reactivation in pts with STs and the treatment outcomes in pts with HBV reactivation have not yet been fully elucidated. Therefore, we prospectively investigated the incidence of HBV reactivation in pts with STs receiving first-line chemotherapy and the clinical outcomes of these patients. Methods: ST pts with seropositivity for HBsAg [sAg(+)], or seronegativity for HBsAg and seropositivity for HBcAb and/or HBsAb [c/sAb(+)], receiving first-line chemotherapy including neoadjuvant or adjuvant chemotherapy, were enrolled. HBV DNA was at least measured at the termination of the first-line chemotherapy, and at 3, 6, and 12 months after the termination of first-line chemotherapy. It was also measured in cases where HBV reactivation was suspected, such as based on increase in the serum levels of transaminases. HBV reactivation was defined as a 10-fold or greater increase in the titer of HBV DNA. Results: A total of 379 pts [sAg(+), 35 pts; c/sAb(+), 344 pts] were enrolled in this study. Any cancer types and regimens were included, and the median period of measurements of HBV DNA was 74 days (range, 13-248 days). Among the pts with sAg(+), HBV reactivation occurred in 9 pts, and the incidence rates at 6, 12 and 18 months were 6.8%, 30.0%, and 35.8%, respectively. Among pts with c/sAb(+), HBV reactivation occurred in 7 pts, and the incidence rates at 6, 12 and 18 months were 0.6%, 2.0% and 3.1%, respectively. Four of the seven pts with c/sAb(+) showing HBV reactivation improved without antiviral therapy, and there were no pts with increased serum transaminase levels or fulminant hepatitis in this series. Conclusions: HBV reactivation in ST pts with c/sAb(+) was rare, and no case with clinically significant HBV reactivation was detected by periodic measurement of HBV DNA and proper management at the first sign of reactivation. Clinical trial information: UMIN000005369.