Matrix metalloproteinases play an essential role in tumor growth and invasion. Different matrix metalloproteinases are often expressed in cancers with distinct patterns. To investigate the role of human macrophage metalloelastase (MMP-12) in epidermal tumors, we studied human macrophage metalloelastase mRNA and protein expression in malignant squamous cell and basal cell carcinomas, and in premalignant Bowen's disease. Human macrophage metalloelastase was detected in 11 of 17 squamous cell carcinomas in epithelial cancer cells, whereas macrophages were positive in 15 of 17 samples. In basal cell carcinomas, human macrophage metalloelastase was more often found in macrophages (seven of 19) than in cancer cells (four of 19). Human macrophage metalloelastase mRNA was also detected in three cell lines derived from squamous cell carcinomas of the head and neck and in transformed HaCaT cells, whereas premalignant tumors and primary keratinocytes were negative for human macrophage metalloelastase mRNA. Western analysis revealed human macrophage metalloelastase protein in squamous cell carcinoma cells. Our results show that human macrophage metalloelastase can be expressed in vivo and in vitro by transformed epithelial cells and indicate that the level of human macrophage metalloelastase expression correlates with epithelial dedifferentiation and histologic aggressiveness.