We have recently reported the presence and a potential proinflammatory role of IL-18 in the synovium of patients with rheumatoid arthritis. To obtain direct evidence that IL-18 plays an influential role in articular inflammation, we investigated the development of collagen-induced arthritis in a strain of mice lacking IL-18 (IL-18−/−) of DBA/1 background. IL-18−/− mice developed markedly reduced incidence of arthritis compared with heterozygous or wild-type mice. Of the IL-18−/− mice that developed arthritis, the severity of the disease was significantly reduced compared with the intact mice. This was accompanied by reduced articular inflammation and destruction evident on histology. IL-18−/− mice also had significantly reduced Ag-specific proliferation and proinflammatory cytokine (IFN-γ, TNF-α, IL-6, and IL-12) production by spleen and lymph node cells in response to bovine type II collagen (CII) in vitro compared with wild-type mice, paralleled in vivo by a significant reduction in serum anti-CII IgG2a Ab level. Treatment with rIL-18 completely reversed the disease of the IL-18−/− mice to that of the wild-type mice. These data directly demonstrate a pivotal role of IL-18 in the development of inflammatory arthritis and suggest that antagonists to IL-18 may have therapeutic potential in rheumatic diseases.