OBJECTIVE To determine the frequency and clinical significance of high titers of IgG autoantibodies to nuclear lamin B1 in a large number of unselected and well-characterized systemic lupus erythematosus (SLE) patients, disease controls, and normal healthy controls. METHODS A cross-sectional study of anti-lamin B1 autoantibodies, as measured by enzyme-linked immunosorbent assay using human recombinant lamin B1 autoantigen, was performed on serum samples obtained at first evaluation of 238 consecutive French Canadian adults: 61 healthy control subjects, 20 patients with osteoarthritis, 22 with ankylosing spondylitis, 11 with autoimmune hepatitis, 30 with rheumatoid arthritis, and 94 with SLE. SLE patients were studied for 57 disease manifestations. A case-control study was performed to analyze the relationship between anti-lamin B1 status and thrombotic manifestations between SLE onset and last followup. RESULTS High titers of anti-lamin B1 were strikingly restricted to a subset of 8 SLE patients (8.5%). The mean anti-lamin B1 titer was higher in this subset than in the other SLE patients or any control group (P<0.001). By univariate analysis and stepwise multiple logistic regression, the most striking association of anti-lamin B1 was with lupus anticoagulant (LAC) antibodies (P = 0.00001). Although LAC were significantly associated with thrombosis in our SLE patients, anti-lamin B1 was not. The frequency of thrombosis in SLE patients expressing both LAC and anti-lamin B1 was similar to that in patients without LAC (P = 1.0). However, patients expressing LAC without anti-lamin B1 had a greater frequency of thrombosis (P = 0.018). CONCLUSIONS High titers of IgG anti-lamin B1 autoantibodies are highly specific for a subset of SLE patients whose clinical characteristics include the presence of LAC and other laboratory manifestations of the antiphospholipid syndrome. The presence of LAC without anti-lamin B1 may define a subset of SLE patients at greater risk for thrombosis.