New cancer therapies are being sought that target both the tumour cells and the surrounding microenvironment. Butyrate is thought to be an HDAC inhibitor and has been shown to cause cell cycle arrest and apoptosis in colon cancer. More recently studies have suggested that butyrate can also down-regulate the pro-angiogenic factor VEGF in colon cancer. Therefore this study aimed to test the hypothesis that butyrate will stimulate breast cancer cell apoptosis and concomitantly inhibit angiogenic factor expression. Treatment of MCF-7 and MDA-MB-231 breast cancer cells with increasing concentrations of butyrate (0-20mM) resulted in a significant decrease in breast cancer cell proliferation (P<0.005) as measured by MTS assay (EC50 3mM), cell cycle arrest in G1 (P<0.04), and a significant increase in apoptosis as measured by BAK expression using high content analysis (P<0.001). Protein array analysis following 48 hours of treatment with 5mM butyrate demonstrated a significant (P<0.05) decrease in the anti-apoptotic proteins survivin, XIAP, CIAP-1 and CIAP-2 and an increase in the pro-apoptotic p21 and p27 proteins. Analysis of angiogenesis related proteins revealed that the pro-angiogenic factors HIF1-a, VEGF, PDGF-AA and PDGF-AB/BB were consistently down-regulated, there was little change in the angiopoietins and that angiogenin was up-regulated. Further analysis revealed that both VEGF mRNA and protein were significantly (p<0.02) down-regulated in MDA-MB-231 cells in a dose dependent manner following butyrate treatment. These data suggest that butyrate will have dual effects as a treatment for breast cancer; causing apoptosis of the breast cancer cells and indirectly inhibiting angiogenesis. Work is currently underway to establish whether butyrate will also be efficacious under hypoxic conditions.