The antiestrogen tamoxifen citrate is an effective antitumor agent in postmenopausal women with advanced breast cancer. The drug has produced relatively few and generally mild side effects. However, a not uncommon clinical phenomenon that may falsely suggest premature discontinuation of tamoxifen therapy has become evident to us and has not yet been sufficiently emphasized in the literature. We have designated this phenomenon as the tamoxifen flare. It consists of transient, at times severe, increase in pain with an apparent worsening of the patient's clinical status occurring within the first few weeks of therapy. In each of six patients experiencing this flare (45 patients treated) pain subsided despite continuation of tamoxifen therapy, followed by a partial remission lasting from six to 20 months.