24-hour mean plasma levels of T3 and T4 were compared in 29 rigorously selected breast cancer patients (all stages) and 27 healthy women, and 8 A.M. "spot" plasma T4 levels were compared in 43 consecutive unselected breast cancer patients (all stages), 22 women with other-than-breast cancer, 21 women with miscellaneous non-cancerous illnesses, and the same 27 healthy women. The 24-hour T3 levels were the same in the breast cancer patients and healthy controls, but the 24-hour T4 levels were significantly higher in the cancer patients (7.7 vs 5.8 micrograms/dl, p less than 0.001); equal elevations were present in all stages of cancer. Spot T4 levels were likewise significantly higher in the breast cancer patients than in the healthy controls (8.8 vs 7.3 micrograms/dl, p less than 0.005). The women with other-than-breast cancer and the women with miscellaneous non-cancerous illnesses also showed significant elevations of spot T4 levels, indistinguishable from those of the breast cancer patients. It is concluded that breast cancer patients as a group show significant hyperthyroxinemia and that this finding may represent a second nonspecific abnormality of thyroid hormones in disease, hypotriiodothyroninemia (low-T3 syndrome) being the first.