Objectives. The objectives of this study are to examine the efficacy and safety of low-dose aspirin (LDA) vs LDA plus low-intensity warfarin (LDA + W) in the primary thrombosis prevention of aPL-positive patients with SLE and/or obstetric morbidity and the role of clinical and serological markers in the development of thrombosis.Methods. In this 5-year prospective, randomized, open, controlled trial, 166 patients with aPL were randomly assigned using a minimization protocol to receive treatment with LDA (n = 82) or LDA + W [international normalized ratio (INR) = 1.5] (n = 84). Sixty-six patients who declined randomization were followed up in an observational arm. Clinical and laboratory characteristics and medication side effects were recorded.Results. There were no differences in the number of thromboses between patients treated with LDA (4/82) or LDA + W (4/84) [hazard ratio (HR) 1.07, 95% CI 0.27, 4.3]. The incidence of thrombosis in the randomized patients was 8/166 (1.8 events/100 person-years) (HR 1.07, 95% CI 0.27, 4.3) and in the observational arm was 7/66 (4.9 events/100 person-years) (HR 2.43, 95% CI 0.87, 6.79). Sixty-five of 66 patients included in the observational arm received LDA. None of the examined clinical or serological factors appeared to predict thrombosis. Medication side effects included mild gastrointestinal symptoms in the LDA group (n = 2) and bleeding in the LDA + W group (n = 11; 1 nasal and 10 menorrhagia). The risk difference for bleeding was 13% (CI 6, 20).Conclusion. No differences in the number of thromboses were observed between patients treated with LDA vs those treated with LDA + W. More episodes of bleeding were detected in the LDA + W group. The LDA + W regime was significantly less safe and not as acceptable as LDA alone. Trial registration: ISRCTN81818945; http://isrctn.org/. © The Author 2013. Published by Oxford University Press on behalf of the British Society for Rheumatology. All rights reserved.