OBJECTIVE: To assess whether temperature is increased by medroxyprogesterone (MPA) and thus whether basal temperature records could be used to determine ovulation during cyclic MPA therapy. DESIGN: A 2-month double-blind placebo-controlled crossover trial in which oral basal temperature was measured daily. SETTING: Normal human volunteers in an academic medical environment. SUBJECTS: Eleven postmenopausal women not taking gonadal hormones. INTERVENTION: Medroxyprogesterone acetate (10 mg/d) or placebo, calendar days 16 to 25, with crossover. MAIN OUTCOME MEASURES: Comparison of mean temperature days 17 to 26 during MPA versus placebo; comparison of differences between temperatures days 7 to 16 and 17 to 26 in MPA versus placebo months; and analysis for a significant monthly thermal shift. RESULTS: The mean temperatures during MPA treatment averaged 0.27 degrees C higher than during the placebo phase and showed a significant change from pretreatment to "treatment" phases during MPA but not during placebo cycles. Eight of the MPA and one of the placebo cycles showed a shift from lower to higher temperatures days 16 to 25. CONCLUSIONS: Medroxyprogesterone acetate has a physiological progesterone-like thermal effect. Therefore basal temperature data cannot reliably indicate ovulation during cyclic MPA administration.