OBJECTIVE: To study the effect of E2 and P on the epithelial cell cycle of normal human breast in vivo. DESIGN: Double-blind, randomized study. Topical application to the breast of a gel containing either a placebo, E2, P, or a combination of E2 and P, daily, during the 10 to 13 days preceding breast surgery. PATIENTS: Forty premenopausal women undergoing breast surgery for the removal of a lump. MAIN OUTCOME MEASURES. Plasma and breast tissue concentrations of E2 and P. Epithelial cell cycle evaluated in normal breast tissue areas by counting mitoses and proliferating cell nuclear antigen immunostaining quantitative analyses. RESULTS: Increased E2 concentration increases the number of cycling epithelial cells. Increased P concentration significantly decreases the number of cycling epithelial cells. CONCLUSION: Exposure to P for 10 to 13 days reduces E2-induced proliferation of normal breast epithelial cells in vivo.