Anti-prothrombin (anti-PT) antibodies are recently identified antibodies directed toward a phospholipid-binding protein (prothrombin), which deserve attention for the reported clinical and pathogenic relevance in antiphospholipid syndrome and systemic lupus erythematosus (SLE). We have recently carried out a longitudinal study on the predictive value of anti-PT antibodies in SLE showing that they have a higher diagnostic accuracy for thrombosis than anti-beta(2)-GPI and anticardiolipin antibodies, and, along with LAC activity, are the best predictors of thromboembolic events in SLE patients.