Objective: To determine whether the use of oral micronized progesterone (OMP) to induce withdrawal bleeding in women suspected of having polycystic ovary syndrome (PCOS) alters circulating androgen levels. Design: Prospective clinical trial. Setting: Academic medical center. Patient(s): Eight reproductive-aged women with PCOS. Intervention(s): Blood was sampled before (week 0) and weekly after (weeks 1 to 4) the administration of OMP (Prometrium, Solvay Pharmaceuticals, Marietta, GA), 100 mg in the morning and 200 mg before bedtime for 7 days. Main Outcome Measure(s): The levels of total testosterone (TT), free testosterone (FT), sex hormone-binding globulin (SHBG), dehydroepiandrosterone sulfate (DHEAS), and androstenedione (A4) were determined in the blood samples. Result(s): In seven of the eight women studied, menstrual cycle intervals were >3 months, while one was eumenorrheic; six of the eight women had hirsutism (modified Ferriman-Gallwey score >7). Mean age was 28.9 ± 10.4 years and mean body mass index was 33.9 ± 4.7 kg/m2. The mean values of TT, FT, SHBG, DHEAS, A4, and 17-OHP did not change with OMP administration. However, a higher 17-OHP level was observable at the completion of OMP administration (week 2). Conclusion(s): We conclude that the administration of OMP (100 mg in the morning and 200 mg before bedtime for 7 days) to induce withdrawal bleeding in women with PCOS does not significantly alter circulating androgen or 17-OHP levels, and can be used to time blood sampling in these patients. © 2002 by American Society for Reproductive Medicine.