Progesterone is the most widely used treatment for premenstrual syndrome. To answer definitely the question of whether progesterone suppositories are effective for the treatment of premenstrual syndrome, a randomized, placebo-controlled, double-blind crossover study of 168 women, receiving progesterone in doses of 400 and 800 mg or placebo, was carried out. Premenstrual symptoms were not significantly improved by progesterone compared with placebo in any measure used in the study, including daily symptom reports maintained throughout treatment, clinician evaluation of improvement, and patient global reports of symptom severity, relief, and disruption of daily activity. No symptom cluster or individual symptom differed significantly between progesterone and placebo treatment. These treatment results were not significantly affected by fluctuations in response during the placebo washout period, pretreatment levels of depression or anxiety at either postmenstrual or premenstrual times, or any of 19 other background, medical history, or symptom variables examined individually as covariates with treatment.