PURPOSE: To investigate the short-term biological effects of neoadjuvant chemotherapy +/- zoledronic acid (ZOL) in invasive breast cancer Experimental design: Forty patients were randomised to receive a single 4mg infusion of ZOL 24 hours after the first cycle of FE100C chemotherapy, or chemotherapy alone. Randomisation was stratified for tumour stage, ER, HER2, and menopausal status. All patients had repeat breast core-biopsy at Day 5 (D5) +/- Day 21 (D21). Effects on apoptotic index, proliferation (Ki67), growth index, surrogate serum markers of angiogenesis (VEGF) and serum reproductive hormones within the TGFbeta family (activin-A, TGFbeta1, inhibin-A and follistatin) were evaluated and compared. RESULTS: Baseline clinico-pathological characteristics were well balanced. Cell growth index (increased apoptosis and reduced proliferation) fell at D5 in both groups but recovered more rapidly with chemotherapy+ZOL than chemotherapy alone by D21 (p=0.006). At D5, a greater reduction in serum VEGF occurred with chemotherapy+ZOL compared to chemotherapy: median percentage change -23.8% (IQR -32.9, -15.8) vs. -8.4% (IQR -27.3, +8.9; p=0.02), but these effects were lost by D21. Postmenopausal women demonstrated a decrease in follistatin levels from baseline in the chemotherapy+ZOL group at D5 and D21, compared to chemotherapy alone (interaction p=0.051). CONCLUSIONS: In this pilot study, short-term changes in biomarkers suggest potentially relevant interactions between tumour biology, chemotherapy, modification of the bone microenvironment and the endocrine status of the host. Larger studies with more frequent dosing of zoledronic acid are needed to assess these complex interactions more thoroughly.