We have prospectively tested the effects of simvastatin on the pharmacokinetics of anastrozole and on estrogen concentrations in postmenopausal women with hormone receptor-positive breast cancer who were receiving adjuvant anastrozole. Following 14 days of simvastatin, we did not observe a significant change in plasma concentrations of anastrozole or hydroxyanastrozole in nine evaluable women. Likewise, we did not observe any statistically significant change in serum concentrations of either estradiol, which remained in the undetectable range, or estrone sulfate. Simvastatin and anastrozole may be safely co-administered. Pharmacokinetic results suggest that simvastatin is not likely to compromise the activity of anastrozole.