The present study evaluates the potential beneficial effect of co-treatment with LHRH-agonist in resolving premenopausal tamoxifen's induced supraphysiological serum 17beta estradiol levels and persistent ovarian cysts. Ultrasonographic and serum hormonal evaluations were performed before, during, and following three consecutive injections of long acting LHRH-agonist administered to 14 premenopausal breast cancer patients treated with tamoxifen, who had supraphysiological serum 17beta estradiol levels and simultaneous persistent ovarian cysts. Within 3 weeks of the first LHRH-agonist injection, all patients had menopausal serum estradiol levels. Ovarian cysts completely disappeared within 2 months following the first injection. Following the discontinuation of LHRH-agonist co-treatment, serum estradiol levels remained in physiological levels and the ovaries remained a normal size in 64.3% of the patients for 13.3 +/- 11.5 months. 28.6% of the patients had a gradual reappearance of high serum estradiol levels and of ovarian cysts, and were, therefore, treated with a second course of LHRH-agonist. Following the second course, serum estradiol levels remained in physiological levels and the ovaries remained a normal size for 8-15 months. It is concluded that short duration of co-treatment with long acting LHRH-agonist administered to premenopausal breast cancer patients treated with tamoxifen, successfully resolved the tamoxifen-induced supraphysiological serum 17beta estradiol levels and the ovarian cysts.