Understanding cytokine immunobiology is central to the development of rational therapies for destructive inflammatory diseases such as rheumatoid arthritis (RA) and periodontitis. The classical interleukin-1 (IL-1) family cytokines, IL-1α and IL-1β, as well as IL-18, play key roles in inflammation. Recently, other members of the IL-1 family have been identified. These include six cytokines whose genes are located downstream of the genes for IL-1α and IL-1β on chromosome 2 (IL-1F5-10) and also IL-33, which is the ligand for ST2, a member of the IL-1R/Toll-like receptor (TLR) receptor superfamily. IL-1F6, IL-1F8 and Il−1F9 are agonists and, along with their receptor IL-1Rrp2, are highly expressed in epithelial cells suggesting a role in immune defence in the skin and the gastrointestinal (GI) tract including the mouth. Synovial fibroblasts and articular chondrocytes also express IL-1Rrp2 and respond to IL-1F8, indicating a possible role in RA. IL-33 is associated with endothelial cells in the inflamed tissues of patients with RA and Crohn's disease, where it is a nuclear factor which regulates transcription. IL-33 is also an extracellular cytokine: it induces the expression of T helper 2 (Th2) cytokines in vitro and in vivo as well as histopathological changes in the lungs and GI tract of mice. Therapeutic agents which modify IL-1 cytokines (e.g. recombinant IL-1Ra) have been used clinically and others are at various stages of development (e.g. anti-IL-18 antibodies). This review highlights the emerging data on these novel IL-1 cytokines and assesses their possible role in the pathogenesis and therapy of destructive inflammatory disorders such as RA and periodontitis.