Preclinical studies have provided evidence that matrix metalloproteinases (MMPs), a family of zinc-containing proteolytic enzymes, facilitate tumor invasion, the establishment of metastases, and the promotion of tumor-related angiogenesis. Matrix metalloproteinase inhibitors (MMPIs) have been shown to inhibit tumor growth and dissemination in preclinical models. Not all lung cancers express the MMPs believed to be most important in promoting the neoplastic process, and there are conflicting reports regarding the prognostic significance of MMPs in lung cancer. However, it is possible that these observations are because of limitations in the procedures for measuring MMPs. Many investigators believe that MMPs are universally involved in tumor progression; this hypothesis was the basis for initiating seven phase III MMPI trials in lung cancer. Four studies were closed at completion of the predefined accrual goal, and three were closed early. There were no significant differences in survival in a non-small cell lung cancer prinomastat study, and in a small cell lung cancer marimastat trial. The results of the remaining five studies have not been reported. At this point it appears that MMPIs will probably not play a major role in the treatment of advanced lung cancer patients.