Background and purpose: Cholesterol-lowering drugs (statins) appear to have pleiotropic effects independent of cholesterol level. The aim of this study was to assess the effect of pre-operative therapy with a statin (atorvastatin) on tumor proliferation in patients with newly diagnosed breast cancer. Methods: Thirty cases with histologically proven breast cancer were subjected to treatment by atorvastatin 80 mg /day for at least 14 days before the final surgical procedure (MRM or BCS). Immunohistochemical expression of Ki-67 staining of breast tumor cells was evaluated to assess tumor proliferation in biopsy tissues before treatment with atorvastatin then in final surgical tissues specimens. Results: The median age of the patients was 49.5 ranging from 29 to 61 years. The vast majority of the patients had invasive duct carcinoma (IDC) and was positive for estrogen and progesterone receptors. The mean pretreatment Ki67 index was high in the majority of patients and was significantly associated with both tumor grade and estrogen receptor status (P = 0.001 and P= 0.003, respectively). The Ki67 index had decreased in the post-treatment samples after final surgery in 19 cases, increased in 8 cases and unchanged in 3 cases compared to the pre-treatment specimens. Tumor grade is a significant predictor of treatment response (p=0,05). Conclusion: Atorvastatin decreased tumor proliferation in breast cancer especially in high grade tumors and its role should be considered in the future studies.