A new immunohistochemical assay using a monoclonal estrogen receptor (ER) antibody (H222, Abbott Laboratories, North Chicago) for determination of ER in formalin-fixed paraffin-embedded tissue was applied to evaluate its clinical value in a group of 145 previously untreated patients with advanced breast cancer. Suitable histologic material was accessible in 137 of these patients, of whom 70% had ER-positive tumors. The ER-positive patients had a significantly longer median overall survival than ER-negative patients (67 versus 32 months, P much less than 0.001) and this was an effect of both a prolonged disease-free interval (27 versus 17 months, P less than 0.05) and a prolonged survival after recurrence (41 versus 15 months, P much less than 0.001). Response to endocrine therapy was obtained in 49% of the patients with ER-positive tumors and in 7% with ER-negative tumors (P much less than 0.001). Relationship between response and semiquantified individual staining features could not be established. It is concluded that ER analysis in formalin-fixed paraffin-embedded tissue offers clinically useful information for allocation of patients to endocrine therapy.