In a hospital-based case-control study conducted in New Haven, Connecticut, women experiencing estrogen replacement therapy were found to be at twice the risk of nonusers for histologically confirmed fibrocystic breast disease (odds ratio = 2; 95 percent confidence limits = 1-3.9) if their menopause was natural. No excess risk was found for women experiencing a surgical menopause. The highest risk for fibrocystic disease was observed for women with more than three years of estrogen replacement therapy. When therapy occurred was not significantly related to the risk of disease once duration of use was controlled for. These results suggest an etiologic role of estrogen replacement therapy in the development or promotion of fibrocystic breast disease.