An interview study of 1362 breast cancer cases and 1250 controls identified through a multi-center screening program allowed evaluation of effects of thyroid disease and supplementation on breast cancer risk. A previous diagnosis of treated thyroid disease was not associated with an excess risk (RR = 1.0), nor were any specific diagnoses, including hypothyroidism, hyperthyroidism, or goiter. Although based on limited numbers, women with untreated hypothyroidism or goiter had a significantly reduced risk of breast cancer (RR = 0.3, 95% CI 0.1-0.7). Thyroid supplementation for non-disease reasons (primarily weight loss and fertility problems) was associated with a slight elevation in breast cancer risk (RR = 1.2, 95% CI 0.9-1.7), but patterns of risk by duration and latency generally failed to provide evidence of causality. Elevated risks were noted among women who received thyroid medications for fertility problems (RR = 4.2) and among those with a family history of breast cancer (RR = 2.6) or a late age at first childbirth (RR = 2.4), possibly indicating an hormonal interaction.