We studied the prevalence and clinical significance of anti-U1 RNA antibodies in patients with systemic sclerosis. The presence of anti-U1 RNA antibodies was determined using immunoprecipitation in systemic sclerosis patients with anti-U1 RNP antibodies (n=36), antitopoisomerase I antibodies (n=20), or anticentromere antibodies (n=20), mixed connective tissue disease patients (n=23), systemic lupus erythematosus patients with anti-U1 RNP antibodies (n=26), and normal controls (n=20). Moreover, antigen specificities for anti-U1 RNP antibodies were examined in patients with systemic sclerosis by immunoblotting and enzyme-linked immunosorbent assay. Anti-U1 RNA antibodies was detected in 22 of 36 systemic sclerosis patients (61%) with anti-U1 RNP antibodies, 14 of 23 patients (61%) with mixed connective tissue disease, and eight of 26 systemic lupus erythematosus patients (31%) with anti-U1 RNP antibodies. Anti-U1 RNA antibodies were not detected in other groups. As for systemic sclerosis patients, the frequencies of pulmonary fibrosis and reduced percentage diffusion capacity for carbon monoxide were significantly greater in patients with anti-U1 RNA antibodies than in those without (76%vs 18%, p<0.005; 82%vs 27%, p<0.005, respectively). Moreover, patients with anti-U1 RNA antibodies had significantly lower percentage diffusion capacity for carbon monoxide and percentage vital capacity values than those without (51.9+/-16.8 vs 79.4+/-16.4, p<0.01; 83.8+/-21.4 vs 101.4+/-12.9, p<0.05, respectively). Regarding the antigen specificities of anti-U1 RNP antibodies in systemic sclerosis patients, the frequency of anti-70 kDa antibodies determined by immunoblotting was significantly higher in patients with anti-U1 RNA antibodies than in those without (77%vs 43%, p<0.05). This finding was also confirmed by enzyme-linked immunosorbent assay for anti-70 kDa antibodies (86%vs 43%, p<0.05). These results indicate that anti-U1 RNA antibodies may be a serologic indicator for pulmonary fibrosis in systemic sclerosis patients with anti-U1 RNP antibodies.