The antiestrogen tamoxifen citrate is an effective antitumor agent in postmenopausal women with advanced breast cancer. The drug 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 and has not yet been sufficiently emphasized in the literature. This phenomenon was designated 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 1st few wk of therapy. In each of 6 patients experiencing this flare (45 patients treated) pain subsided despite contination of tamoxifen therapy, followed by a partial remission lasting from 6-20 mo.