The estrogen rebound response in metastatic breast cancer has been reported in the past as a rare and short-lived phenomenon, not clearly associated with any aspect of the patient's clinical profile. In this series, 25% of patients responding to estrogen therapy had a further response when they no longer received the hormone. The median duration of this palliation was a minimum of 18 months and was similar to that of the initial estrogen response. Patients with a rebound response had significantly longer disease-free intervals before estrogen therapy and estrogen remissions than those who did not have a rebound response. These clinical factors may, therefore, be helpful in predicting the chances of a rebound response in any given patient. It is urged that patients be observed for objective improvement without therapy upon escape from estrogen palliation. Further palliative therapy should not be offered until definite progression of disease has been documented after any rebound response that occurs.