Objective: Our goal was to evaluate the efficacy and side effects of danazol 200 mg daily given only in the luteal phase of the menstrual cycle to treat premenstrual syndrome and premenstrual mastalgia. Study Design: We conducted a randomized, double-blind, placebo-controlled study of 3 menstrual cycles in a postgraduate medical school and National Health Service hospital. The subjects of the study were 100 women who had been referred to the premenstrual syndrome clinic at the North Staffordshire Hospital for the management of premenstrual syndrome and premenstrual breast pain. Outcome measures for the study included assessment of improvement in symptoms measured by specific daily visual analogue scales for 4 principal symptoms of premenstrual syndrome and for premenstrual mastalgia and assessment of side effects and adverse events. Results: Significant improvement in symptoms was seen in visual analog scores for mastalgia in months 1 ( P = .03), 2 ( P = .004), and 3 ( P = .01) of the study during active therapy compared with placebo. No improvement was seen for any other symptom or for the global premenstrual syndrome score. Side effects on danazol and on placebo were equal and minimal. Conclusions: Luteal phase–only danazol is not effective for the treatment of the general symptoms of premenstrual syndrome but appears highly effective for the relief of premenstrual mastalgia. This approach to therapy is associated with few side effects. Studies of cyclic mastalgia using strict diagnostic criteria are required to see whether the freedom from symptomatic side effects is found in longer-term studies and to determine whether such a regimen avoids potentially detrimental effects on the lipid status. (Am J Obstet Gynecol 1999;180:18-23.)