Studies from model systems suggest that matrix metalloproteinases (MMPs) are causally involved in tumor progression while tissue inhibitors of MMPs (TIMPs) prevent this progression. Here, we show that concentrations of TIMP-1 are significantly higher in breast carcinomas than in fibroadenomas. In primary breast cancers, TIMP-1 concentrations increased with increasing tumor size but showed an inverse relationship with estrogen receptor concentrations. In primary breast cancers also, TIMP-1 levels were weakly but significantly correlated with those for MMP-1, proMMP-2, active MMP-2, MMP-3 and proMMP-9. Contrary to what might be expected from published data on model systems, high concentrations of TIMP-1 predicted a poor outcome in patients with breast cancer. We conclude that in human breast cancer, endogenous TIMP-1 does not inhibit tumor progression but may enhance the process.