OBJECTIVE To determine whether the anti-68 kDaU1snRNP antibody is associated with mixed connective tissue disease and not with SLE; to evaluate correlations between anti-U1snRNP titers and disease activity; and to look for associations between anti-U1snRNP specificities and specific clinical features. METHODS 40 patients with a positive double diffusion test for anti-68 kDa U1snRNP were studied, including 21 with mixed connective tissue disease, 14 with systemic lupus erythematosus and five with other connective tissue diseases. IgGs to 68 kDa U1snRNP were assayed using an ELISA. Clinical features, disease activity and antibody test findings were evaluated longitudinally in nine patients. RESULTS Both proportions of patients with anti-68 kDa U1snRNP and titers of IgG to 68 kDa U1snRNP were similar in the mixed connective tissue disease and systemic lupus erythematosus groups. Patients with mixed connective tissue disease were significantly more likely to have anti-A U1snRNP or anti-C U1snRNP than those with systemic lupus erythematosus (P < 0.03 and P < 0.04, respectively). No significant correlations were found between any of the features of mixed connective tissue disease (e.g., Raynaud's phenomenon, myositis, or sausage digits) and a specific anti-U1snRNP antibody. During follow-up (mean, seven years; range, 1-25 years), changes occurred in the anti-U1snRNP profile and in the anti-68 kDa U1snRNP titer. These changes were not correlated with disease activity. CONCLUSIONS IgGs to 68 kDa U1snRNP are not associated with a specific pattern of anti-RNP-positive connective tissue disease. No useful information can be gained by monitoring anti-68 kDa U1snRNP IgG titers over time. A Western blot profile including anti-A U1snRNP or anti-C U1snRNP indicates a high likelihood of U1snRNP-associated mixed connective tissue syndrome (MCTD).