The objective was to determine the serum levels of manganese superoxide dismutase (Mn SOD) in patients with localized scleroderma and investigate their clinical significance in this disease. Serum samples from 15 patients with localized scleroderma and 20 healthy volunteers were examined by a specific enzyme-linked immunosorbent assay. Serum levels of Mn SOD were significantly higher in patients with generalized morphea than those in healthy individuals. And the patients with elevated serum Mn SOD levels had significantly larger number of sclerotic lesions and significantly higher serum levels of soluble interleukin-2 receptor than those without it. These results suggested that the serum levels of this enzyme may be a serological marker for the disease activity and the extent of skin involvement in this disease.