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Background and Objectives: The objective of this study was to evaluate the prevalence, clinicopathologic features, and outcome of renal involvement in a large cohort of patients with primary antiphospholipid syndrome (PAPS)., Design, Setting, Participants, & Measurements: We retrospectively examined medical records of 160 patients with a diagnosis of PAPS of two general hospitals of northern Italy between 1985 and 2008., Results: There were 140 women and 20 men. Mean age was 35+/-12 yr. PAPS was characterized by thrombotic events in 41.2%, fetal loss in 39.4%, and both in 19.4%. Signs of renal abnormalities were present in 14 (8.7%) patients. All patients had proteinuria, in the nephrotic range in five; four patients had moderate chronic renal insufficiency, and one had end-stage kidney disease (ESKD). Two patients presented with acute renal failure and one with nephritic syndrome. Ten patients underwent a renal biopsy, which showed a membranous glomerulonephritis in four, proliferative glomerulonephritis in two, thrombotic microangiopathy in two, and vascular lesions consistent with chronic antiphospholipid antibodies nephropathy in two. Patients with renal involvement were older (41.8 versus 34.3 years; P=0.0269), more frequently lupus anticoagulant positive (92.3 versus 48.9%; P=0.0068), and had hypocomplementemia (P<0.05)., Conclusions: Renal abnormalities are present in approximately 9% of patients with PAPS. In addition to APS nephropathy, the prevailing picture is membranous nephropathy. Outcome and long-term follow-up usually are good. Not all of the clinical manifestations of PAPS can be ascribed to thrombotic mechanisms. The heterogeneity of renal involvement confirms the presence of a continuum between systemic lupus erythematosus and PAPS.