Background/Aim: To evaluate amino acid changes within HBV pol/RT after 96 weeks of treatment with TDF or FTC/ TDF and determine their potential association with TDF resistance. Methods: In Study GS-US-174-0121, 280 patients receiving lamivudine (LAM) with detectable LAM-resistance mutations in HBV pol/RT (LAM-R: rtM204V/I±rtL180M) were randomized 1:1 to receive blinded treatment with TDF or FTC/TDF for 96 weeks. Virologic breakthrough (VB) was defined as confirmed HBV DNA >1 log10 increase from nadir or HBV DNA ≥400 copies/mL (69 IU/mL) after <400 copies/mL. Resistance genotyping by HBV pol/RT sequencing was attempted for all patients at baseline and if viremic (HBV DNA ≥400 copies/ mL) at Week 96/study discontinuation. Results: Overall, 18 patients (9 TDF, 9 FTC/TDF) were viremic viremic at Week 96/last visit. The mean baseline HBV DNA was significantly higher for viremic patients (8.04 log10 copies/mL) compared to patients who did not qualify for genotyping (6.39 log10 copies/mL). In the TDF arm, 3 patients had conserved site changes/reversions (1 with VB), 1 had unique polymorphic site changes, 2 had no change, and 3 were unable to be genotyped. In the FTC/TDF arm, 2 patients had conserved site changes/reversions, 1 had unique polymorphic site changes, 4 had no change, and 2 were unable to be genotyped. No phenotypic resistance to TDF was observed. Four of eight (50%) patients had LAM-R reversions (rtV/I204M±rtM180L) on TDF while 1/8 (12.5%) patients on FTC/TDF had LAM-R reversions. Thirteen patients (4.6%) with prior entecavir (ETV) exposure and 25 patients (8.9%) with baseline ETV-R were enrolled; neither had an impact on viral kinetics. Conclusions: No TDF resistance has been detected through 96 weeks of treatment with either TDF or FTC/TDF in LAM-R patients. The presence of ETV-R or ETV exposure did not impact viral kinetics through 96 weeks. Resistance surveillance in this population will continue through Year 5.