OBJECTIVE: To investigate the predictive value of serum miR34a (ser-miR34a) expression for the neoadjuvant chemotherapy (NACT) response and prognosis in breast cancer patients. METHODS: This study included 86 diagnosed stage II/III breast cancer patients and 20 healthy volunteers. Peripheral blood from every participant was collected before the start, at the end of the second cycle, and at the end of NACT. The expression of ser-miR34a was examined by qRT-PCR and its association with the chemotherapy response and prognosis was analyzed. RESULTS: The expression of ser-miR34a in breast cancer patients before NACT was significant higher than that of healthy volunteers. During the NACT, the changes in ser-miR34a expression were significantly associated with treatment response and disease-free survival (DFS). In responding patients, ser-miR34a levels at the end of the second cycle and at the end of NACT were significantly lower than before NACT (P=0.016 and P=0.002, respectively), and in non-responding patients, the changes were insignificant. Survival analyses showed that the patients with decreased ser-miR34a expression from the end of the second cycle and the end of NACT to before NACT had improved DFS compared with that of the patients with increasing ser-miR34a expression (P<0.001 for both). Cox regression analyses showed that the changes of ser-miR34a expression were independent prognostic indicators. CONCLUSIONS: Ser-miR34a is a novel, noninvasive predictive marker for NACT response and prognosis in breast cancer patients.