Background: A potential link between Health-Related Quality of life (HRQoL) and oncologic outcomes such as overall survival or progression-free survival has been underlined for endocrine therapies in patients with metastatic castrate resistant prostate cancer (mCRPC). Other surrogates such as circulating tumor cells (CTCs) or PSA can be used to evaluate disease control. This study explored the associations between HRQoL and biological biomarkers for patients with mCRPC treated by abiraterone / prednisone or prednisone within registration phase III trial COU-AA-301. Methods: Baseline differences of HRQoL evaluated with FACT-P total score (FACT-P TS) according to biological parameters (including CTCs and PSA) and links between HRQoL's change and variations of these parameters were assessed. The primary objective was to estimate the association between improvement or deterioration in FACT-P TS and the variations of CTCs and PSA. All analyses were conducted using clinically meaningful improvement and deterioration in FACT-P TS and subscales. Results: Among 1130 patients enrolled, 1111 (98.3%) had a baseline FACT-P TS available. At baseline, a favorable CTCs count was associated with higher FACT-P TS compared to unfavorable CTCs (difference in means 8 points, [95% CI, 4 to 12] p < 0.001). At 3 months, there were differences in mean change from baseline FACT-P TS favoring patients with biomarkers response, with clinically meaningful difference for CTCs (12.7 points, [95% CI, 6 to 19.5%] p < 0.001) and PSA (11.64 points, [95% CI, 9.3 to 14] p < 0.0001). Biological progression was associated with higher risk of FACT-P TS worsening for PSA (Odds Ratio (OR) 2.8 [95% CI, 1.9 to 4.2]) with more frequent FACT-P TS improvement in case of response for CTCs (OR 3.14 [95% CI, 1.3 to 7.7]) and PSA (OR 2.9 [95% CI 2.1 to 4]). Significantly longer time until definitive deterioration was observed for patients with CTCs or PSA response (p < 0.001) and shorter time in case of progression (p < 0.001). Conclusions: QUA-lify is the first study to show an association between HRQoL and biomarkers outcomes in patients with mCRPC treated with endocrine therapy in a post-taxane setting. This concept is reinforced by the consistency of the association for all analyses carried out.