Acquired antibodies to phospholipids form a heterogeneous group, which may be detected in vitro by the inhibition of phospholipid dependent tests of coagulation (lupus anticoagulant) and also by immunological assays, such that a combined approach is required for their reliable detection. While initially described in sufferers from systemic lupus erythematosus, these antibodies are increasingly recognised in a broad spectrum of disease, most importantly in relation to thromboembolism and recurrent fetal loss; occasionally they may also be found in otherwise healthy individuals. The mechanisms underlying the prethrombotic state associated with these antibodies have not been defined, although interference with the natural anticoagulant systems seems possible. Identification of antiphospholipid in subjects with spontaneous thromboembolism may influence therapeutic decisions, while their presence in women with recurrent fetal loss has lead to attempts to alter the outcome of further pregnancies with anticoagulant and immunosuppressive regimens, however the optimum management has not yet been determined. The recognition of these antibodies and their clinical associations is therefore highly relevant to clinical and laboratory haematology.