IT was early recognized that only a minority of women with breast cancer are sensitive to any method of hormonal control. The proportion varies between 30 and 50 per cent according to age group and whether oestrogen or androgen is administered (Stoll, 1950). It was therefore noteworthy when Loeser (1954) suggested that thyroid administration may protect against recurrence of breast cancer and that the combination of male hormone with thyroid extract yielded greater clinical benefit than the male hormone alone in mammary carcinoma. This suggestion was not followed up by others until Bacigalupo (1959) and Luehrs (1959, 1 960a) and Luehrs and Bacigalupo (1960) reported on the administration of 120 microgrammes daily of tri-iodothyronine (T3) with androgen (Durabolin). They claimed that it led to regression of growth in advanced mammary carcinoma after the androgen alone failed to give a response. More recently Luehrs (1961a, 1961b) further reported that mammary carcinoma which had become unresponsive to oestrogen therapy began to respond again after administration of tri-iodothyronine. This paper reports an attempt to confirm this latter observation in a group of 12 patients with advanced breast cancer.