Localized scleroderma is a connective tissue disorder that is sometimes accompanied by various immunological abnormalities. In this study, we analysed serum levels of rheumatoid factor (RF) isotypes in patients with localized scleroderma and in normal controls to determine if any of these isotypes reflect the severity of the disease. IgM RF, IgG RF and IgA RF were positive in 30%, 21%, and 7% of the patients, respectively. The levels of IgM RF were significantly higher in the patients with generalized morphea (GM), the most severe form of localized scleroderma, than those with linear scleroderma (LS) (P < 0.005) or normal controls (P < 0.0005). The levels of IgG RF were significantly higher in patients with GM than normal controls (P < 0.05). The levels of IgA RF were significantly higher in patients with GM or LS than normal controls (P < 0.001 and P < 0.01, respectively). The count of sclerotic lesions was significantly higher in patients with IgM RF than those without (P < 0.05). These results suggest that the presence of RF isotypes is one of the immunological abnormalities of localized scleroderma. IgM RF seemed to be most useful of these three factors to determine the severity of disease.