Intracellular IL-1 receptor antagonist (icIL-1ra) is reportedly involved in functions independent of blocking IL-1 receptor signaling. Fibroblasts derived from the involved skin of patients with systemic sclerosis (SSc) are predominantly of the myofibroblast phenotype, with higher levels of icIL-1ra compared to normal skin fibroblasts. We examined the effect of overexpression of icIL-1ra on the phenotype and function of normal fibroblasts with respect to the expression of alpha smooth muscle actin (alpha-SMA), a specific marker for myofibroblasts, and plasminogen activator inhibitor (PAI), a protein involved in fibrogenesis and expressed at higher levels in myofibroblasts, and the production of collagenase (matrix metalloproteinase-1 (MMP-1)), the major enzyme involved in the degradation of native collagen in the skin. Normal human foreskin fibroblasts overexpressing icIL-1ra showed higher levels of alpha-SMA and PAI and had lower levels of collagenase and MMP-1 mRNA induced by inflammatory cytokines. By contrast, levels of mRNA for tissue inhibitor of metalloproteinase-1 in the transfected cells were not different from the control cells. Pretreatment of the ic-IL-1ra-transfected cells with antisense oligonucleotide directed against the mRNA of icIL-1ra restored MMP-1 expression induced by stimulation with IL-1beta. Our data indicate novel functions for icIL-1ra, which might be relevant to the genesis of fibrotic diseases such as SSc.