OBJECTIVE To determine if whole blood viscosity (WBV), a rheologic variable contributing to risk of myocardial infarction and stroke in the general population, is elevated in patients with systemic lupus erythematosus (SLE), particularly SLE patients with a history of thrombotic or atherothrombotic events. Because the high rates of arterial and venous thrombosis in lupus cannot be explained by traditional risk factors, elevated WBV may be an easily measurable nontraditional risk factor to identify SLE patients at high risk for thrombotic events. METHODS Sixty SLE patients (30 with a history of a thrombotic event) and 20 matched controls were recruited into the study. The thrombosis group was further subdivided into an arterial thrombosis group (n = 17). WBV values were determined at 9 different shear rates (1, 2, 5, 10, 50, 100, 150, 300, and 1,000 seconds(-1)). WBV was then compared between groups by repeated-measures analysis of variance. RESULTS SLE patients with a history of arterial events had significantly elevated WBV relative to either controls (P = 0.022) or SLE patients without arterial events (P = 0.014). WBV in the total SLE group did not differ from controls. Differences in WBV were most prominent at lower shear rates (1, 2, 5, 10, 50, and 100 seconds(-1)). Anticoagulation, prednisone dose, and antiphospholipid antibodies did not significantly impact WBV. CONCLUSIONS Our study demonstrated that WBV is selectively elevated in patients with SLE with a history of arterial events. Although this association is striking, longitudinal studies are needed to assess the positive predictive value of WBV for atherothrombotic events in SLE.