Abstract purpose: To determine whether the features of the antiphospholipid syndrome (APS) are in any way influenced by the presence or absence of systemic lupus erythematosus (SLE). We followed up patients with ‘primary’ APS (PAPS) and APS secondary to SLE (APS plus SLE) with the objective of comparing laboratory and clinical events and of determining whether patients with PAPS would have evolution to SLE. patients and methods: A total of 114 patients from 3 European referral centers were included in this study. Fifty-six had APS plus SLE and 58 had PAPS. Laboratory and clinical data were collected during an average 2-year period. results: Patients with PAPS and patients with APS plus SLE had similar clinical and laboratory profiles, with the exceptions of autoimmune hemolytic anemia, endocardial valve disease, neutropenia, and low C4 levels, all of which occurred more frequently in patients with APS plus SLE (p values: <0.05, <0.005, <0.01, and <0.001, respectively). On follow-up, 10 thrombotic episodes occurred in 10 patients, 8 of whom were receiving anticoagulant therapy. No patient with PAPS had either anti-DNA or anti-extractable nuclear antigen antibodies, and these patients had a significantly lower prevalence of antinuclear antibodies (41%) than patients with APS plus SLE (89%). conclusions: Patients with APS plus SLE and PAPS have similar clinical profiles, although heart valve disease, hemolytic anemia, low C4 levels, and neutropenia seem to be more common in patients with APS plus SLE. Patients with APS may develop further thrombotic events despite anticoagulation therapy.