Objective. To study the expression and production of interleukin-1-converting enzyme (ICE) in human normal and osteoarthritic (OA) cartilage and synovium, quantitate the level of ICE in OA chondrocytes, and examine the relationship between the topographic distribution of ICE, interleukin-1 (IL-1), and IL-18, as well as apoptosis of chondrocytes. Methods. The expression and synthesis of ICE were investigated in human normal and OA cartilage and synovial membrane using in situ hybridization and immunohistochemical methods. The intracellular level of ICE in OA chondrocytes was also measured by enzyme-linked immunosorbent assay (ELISA). Furthermore, the topographic relationship between the presence of ICE and mature IL-1 and IL-18 was examined by immunohistochemistry, and apoptotic chondrocytes by the TUNEL technique. Results. ICE was expressed and synthesized in both human synovial membrane and cartilage, with a significantly greater number of cells staining positive in OA tissue than in normal tissue. ICE production was preferentially located in the superficial and upper intermediate layers of articular cartilage. With a specific ELISA, a level of 230.2 ؎ ؎ ؎ 22.5 pg/5 ؋ ؋ ؋ 10 5 cells (mean ؎ ؎ ؎ SEM) of ICE was found in OA chondrocytes. In cartilage, IL-1 and IL-18 stained positive at a topographic location similar to that of ICE. The production of mature IL-1 in OA cartilage explants and chondrocytes was completely blocked by treatment with a specific ICE inhibitor, which also markedly diminished the number of IL-18-positive cells. The data show that there was no close relationship between the presence of ICE and the presence of apoptotic chondrocytes in OA cartilage. Conclusion. This study shows, for the first time, the presence of active ICE in human articular cartilage, with a markedly increased cellular level in OA tissue. The relationship between active IL-1 and ICE suggests that ICE may promote OA progression by activating this proinflammatory cytokine. The role of IL-18 in pathologic cartilage is discussed.