The hypothesis that the many non-specific changes normally associated with cyclical ovarian activity are the primary aetiological factors in the premenstrual syndrome was tested by suppressing ovulation with subcutaneous oestradiol implants. Sixty eight women with proved premenstrual syndrome were treated under placebo controlled conditions for up to 10 months in a longitudinal study. Active treatment was combined with cyclical oral norethisterone to produce regular withdrawal periods. Symptoms were monitored with daily menstrual distress questionnaires, visual analogue scales, and the 60 item general health questionnaire. Of the 35 women treated with placebo 33 improved, giving an initial placebo response rate of 94%. The placebo effect gradually waned, but the response to the active combination was maintained for the duration of the study. Analysis of the prospective symptom ratings showed a significant superiority of oestradiol implants over placebo after two months for all six symptom clusters in the menstrual distress questionnaire. Changes seen in the retrospective assessments were less significant but the trend was the same. Treatment with oestradiol implants and cyclical progestogen was well tolerated and appears to be both rational and effective for severe cases of the premenstrual syndrome.