40 patients with premenstrual symptoms were randomly allocated to receive placebo patches or active treatment with transdermal oestradiol patches (2 x 100 micrograms) to suppress ovulation. Norethisterone 5 mg was given in each group from day 19-26 of the cycle to ensure a regular withdrawal bleed. Treatment was for 6 months with crossover at 3 months. Patients completed the Moos menstrual distress questionnaire (MDQ) and the premenstrual distress questionnaire (PDQ) daily throughout the study. 5 patients withdrew, 4 because of skin reactions and 1 because of considerable improvement with initial (active) treatment. After 3 months, both groups showed improvement in MDQ and PDQ scores. In general, between 3 and 6 months, patients who switched from active treatment to placebo had deteriorating scores while patients who switched from placebo to active treatment maintained or improved upon their initial gains. Significant improvements occurred after changing to active treatment in five of six negative MDQ symptom clusters and in six of ten PDQ symptoms.