In examining reasons for premature atherosclerosis in systemic lupus erythematosus (SLE), we previously reported low levels of the cholesterol transport protein apolipoprotein A1 (apoA1) in these patients, and specific antibodies to purified apoA1 were identified in the sera of 5 out of 30 lupus patients. The current study was initiated to determine whether these antibodies are common in lupus patients. 520 serum samples from 175 patients with SLE or primary antiphospholipid syndrome (PAPS) were tested for antibodies to purified apoA1. Positive sera were retested for binding to apolipoprotein incorporated into reconstructed nascent or mature high-density lipoprotein (HDL). Autoantibodies to apoA1 were found in 32.5% of patients with SLE and 22.9% of patients with PAPS, associated with the presence of aPL (anti-beta2 glycoprotein-1, anti-beta2 GP1) antibodies. When reconstructed, nascent and mature HDL molecules were compared as antigen-containing environments, positive sera reacted best to apoA1 embedded in mature HDL molecules. This report confirms the high prevalence of antibodies to apoA1 in patients with systemic lupus and suggests a high affinity of these antibodies for mature HDL.