Premenstrual Syndrome (PMS) is the term for severe symptoms experienced by about 5% of menstruating women up to two weeks before their menstrual periods, but not at other times. Treatment with progesterone may restore a deficiency, or balance the level of progesterone with other menstrual hormones. Progesterone therapy may reduce the effects of falling progesterone levels on the brain or on electrolytes in the blood.The objectives were to determine if progesterone has been found to be an effective treatment for all or some premenstrual symptoms, and if adverse events associated with this treatment have been reported.We last searched the Cochrane Menstrual Disorders and Subfertility Group's Trials Register, the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library, Issue 1, 2005), MEDLINE (1966 to 2005) and EMBASE (1980 to 2005) in March 2005, and PsycINFO (1806 to 2006) in April 2006. We contacted pharmaceutical companies for information about unpublished trials.We included randomised double-blind, placebo-controlled trials of progesterone on women with PMS diagnosed by at least two prospective cycles, without current psychiatric disorder.Two reviewers (BM and OF) extracted data independently, and decided on the trials to be included. OF wrote to the trial investigators to ask for missing data.We considered 17 studies. We included two trials totaling 280 participants aged from 18 to 45 years. Of these 115 yielded analysable results. Both studies measured outcomes using subjective scales of symptom severity but made calculations as if they were interval data. The two studies differed in design, participants, dose of progesterone, how and when the dose was administered and in outcome measures. It was impossible to combine data in a meta-analysis. Adverse events which may or may not have been the side effects of the treatment, were generally described as mild. Both trials intended to exclude women whose symptoms continued after their periods; unfortunately the larger multicentre study had some ineligible participants. Overall, participants benefited more from progesterone than placebo. This was statistically significant in per protocol analysis but not in the intention-to-treat analysis, except for the first cycle. The smaller, crossover study found no statistically significant difference between oral progesterone, vaginally absorbed progesterone and placebo.We could not say that progesterone helped women with PMS, nor that it was ineffective. Neither trial distinguished a subgroup of women who benefited.