Antiphospholipid antibodies (lupus anticoagulant, anti-cardiolipin and anti-beta(2)-glycoprotein I antibodies, mostly IgG isotype) are strong risk factors for thrombosis. Because a paucity of information on IgA isotype exists in the literature, we retrospectively evaluated the thrombotic significance of IgA antiphospholipid antibodies. We included 472 patients with clinical information on thrombotic events and complete laboratory work-up for antiphospholipid antibodies syndrome. Odds ratios (OR) of various antiphospholipid antibodies for thrombosis were calculated by univariate and multivariate analyses. Lupus anticoagulant alone was detected in 57 (12%) patients, ELISA-based antibodies (IgG, IgM, IgA) against cardiolipin, phosphatidylserine or beta(2)-glycoprotein-I alone were detected in 131 (28%) patients, whereas 80 (17%) patients had both. Antibody isotype distribution was IgG 32%, IgM 60% and IgA 56%. Univariate analysis showed a statistically significant risk of thrombosis in patients with elevated titres of IgA of any ELISA-based antiphospholipid antibodies (OR 1.77). Stepwise logistic regression (multivariate) analysis identified elevated titres of any ELISA-based IgA antiphospholipid antibodies as an independent risk factor for thrombosis (OR 1.6) in the entire cohort, and in the subgroup of patients without concurrent presence of lupus anticoagulant (OR 1.8). IgA antiphospholipid antibodies appear to be a significant independent risk factor for thrombosis, thereby meriting evaluation in patients with unexpected thrombosis.