Background: Neoadjuvant therapy (NAT) has now been widely used for patients with operable breast cancer (BC) to test drug sensitivity. Studies have indicated that early assessment of clinical response to NAT can help identify patients who are resistant to initial regimens, providing an additional opportunity to change treatment plan. However, there is a lack of evidence regarding the optimal timing for conducting early evaluations of clinical response. This prospective study was conducted to evaluate the association between early evaluation of clinical response at different timing (post 2 cycles versus post 4 cycles) and pCR post NAT, in order to determine the most appropriate timing for NAT response evaluation. Clinical evaluations using ultrasound and MRI (magnetic resonance imaging) were also compared. Methods: A prospective cohort study including BC patients who received NAT was conducted between January 2018 and December 2023. Multivariable logistic regression analysis was used to estimate the Odds Ratios (OR) and 95% confidence intervals (CI) for the clinical response evaluated at different timings in association with pCR. The heterogeneity of the associations for the post-cycle 2 and post-cycle 4 assessments was assessed. Linear regression analyses were used to assess the associations between extent of tumor reduction post 2 cycles and that post 4 cycles of NAT. Agreement of tumor measurements using ultrasound and MRI was also assessed. Results: A total of 186 BC patients were consecutively enrolled and included in this analysis. 55 (29.6%) of them achieved pCR. Partial response (PR) post 2 cycles and 4 cycles of NAT were both significantly associated with pCR post NAT, though the point estimate of association for PR post 2 cycles of NAT (OR=5.06, 95%CI:2.29-12.09) was more evident than that post 4 cycles (OR=2.79, 95%CI:1.20-7.21). Extent of tumor reduction post two cycles of NAT was highly correlated with that post four cycles (r=0.89, 95%CI=0.79-0.98, P< 0.01). Poor concordance was observed between ultrasound and MRI measurements of tumor size, whether at initial diagnosis (r=0.66, adjusted r²=0.43) or after two cycles of NAT (r=0.59, adjusted r²=0.35). Among those patients who had PR post 2 cycles of NAT, tumor reduction by greater than 50% was not associated with higher probability of reaching pCR in comparison to those lower than 50% (OR=1.86, 95%CI=0.82-4.37, P=0.143). Conclusions: Tumor response assessed using MRI after two cycles of NAT was highly correlated with pCR, and may be adopted as an optimal strategy for early evaluation of the efficacy of NAT regimen. Further studies are warranted to evaluate changes in treatment plan at early phase of NAT for those resistant to initial regimens.