Sixty-three patients with severe breast dysplasia were treated with tamoxifen, 10 mg/day from day 5 to day 25 of the menstrual cycle for 4 months. Lesions were assessed by mammographs, thermography, and echography (A- and B-mode scanning). In 45 patients (71.43%) there was complete remission of symptoms and disappearance of lesions as assessed by echography and clinical examination. Some improvement was seen in 12 patients. In all but 2 of those in whom lesions disappeared completely, no recurrence was seen 12 months after treatment was discontinued. Estrogen receptors were assayed in 10 patients, but were detected in none. A decrease of doubtful statistical significance was observed in some patients in basal and thyrotropin-releasing hormone (TRH)-stimulated levels of prolactin during the treatment period. No other significant or unequivocal change in endocrine parameters was observed during tamoxifen treatment.