OBJECTIVE:Breast cancer is the second most common cancer in Thai women. Current treatments for breast cancer have shown increasing survival rate as well as side effects. This study presents HRQOL outcome of breast cancer treatment using two designs. First, through a cross-sectional study, 167 breast cancer patients receiving care at Khon Kaen Hospital were recruited into three groups: I) newly diagnosed without treatment (n=57), II) undergoing chemotherapy (n=80), and III) completed chemotherapy (n=30). Patients mean age was 51 years (range 31-82) with the majority (76.5%) in early stage. METHODS: HRQOL was assessed using Thai Functional Assessment of Cancer Therapy-Breast (FACT-B) version 4, consisting of five domains: Physical (PWB), Social (SWB), Emotional (EWB) and Functional (FWB) well-beings plus a breast cancer subscale (BCS). Linear regression analysis of QOL scores among the three groups, adjusting for age, disease stage, concomitant disease and hotflush showed patients in group II had lower HRQOL scores than those in group I in most domains, particularly in BCS (beta = -1.6). RESULTS: By contrast, patients group III showed better HRQOL than those in group I and II. HRQOL was greater than before treatment, significant in PWB (beta = 2.2), EWB (beta = 3.4), FACT-G (beta = 8.5) and FACT-B (beta = 9.0). The results of side effects were further evaluated in the second study, where HRQOL was evaluated at the treatment initiation and after six months (N=24). Significantly lower HRQOL scores during the treatment were found. Adjusting for age, disease and side effects, mucositis caused a significant decline in PWB (beta = -12.9) and EWB (beta = -10). CONCLUSIONS:The findings suggest that breast cancer treatment improves patient's HRQOL, with a temporary reduction of HRQOL scores in the breast cancer subscale while receiving treatment. Side effect management, especially for mucositis, could lead to improved HRQOL of these patients.