The excessive deposition of collagen in systemic sclerosis (SSc) is thought to be due to an abnormal function of fibroblasts, which may be the result of an immune dysregulation in skin. Ultraviolet A1 (UVA1) irradiation has been shown to be an effective therapy. This is thought to be due to its capacity to induce matrix metalloproteinase-1 (MMP-1) expression in human dermal fibroblasts. In the present in vitro study, the effect of UVA1 irradiation on MMP-1 was studied using skin fibroblasts from healthy controls (n=5) and patients with systemic sclerosis (n=5). In vitro irradiation studies showed that gene expression for MMP-1 after UVA1 irradiation (p<0.05) was induced in all the fibroblasts studied, but that the induction rate was greater in SSc fibroblasts than in normal ones (p<0.05). The glutathione (GSH) level was lower in SSc fibroblasts than in controls before UVA1 irradiation. However, after UVA1 irradiation, GSH levels were increased and equivalent between normal and SSc fibroblasts. These findings indicated that the relatively stronger increase in MMP-1 expression in SSc fibroblasts was due to the lower antioxidant capacity. These data support the concept that clinical responses to UVA1 radiation are influenced by the antioxidative state of the patients' skin.