The "historical" definition of the antiphospholipid syndrome is the most widely used: history of at least one venous and/or arterial thrombosis and/or two miscarriages, and positive tests for lupus anticoagulant and/or anticardiolipin antibodies, on two occasions more than 8 weeks apart. More recently, a Mexican group proposed another definition taking into account the number of clinical manifestations and the titer of anticardiolipin antibodies to categorize patients as definite, probable or doubtful antiphospholipid syndrome. The respective merits and limitations of both definitions are discussed.