Tamoxifen is an estrogen antagonist/agonist often associated with antiestrogenic effects such as hot flushes and vaginal dryness in premenopausal women. Estrogenic side-effects, such as thromboembolic phenomena and endometrial proliferation has been reported in postmenopausal women. Paradoxically, tamoxifen has also been shown to be capable of increasing estrogen levels in premenopausal women. Since tamoxifen is being used more frequently in this group of women, potential adverse effects are only now being recognized. Two cases of premenopausal women who developed symptomatic endometriomas while on tamoxifen for breast cancer, are reported. Stimulation of endometriosis should be considered when pain and an ovarian mass develops in a woman on tamoxifen. The unique effects of tamoxifen in premenopausal women may contribute to this even in the presence of regular ovulation.