A retrospective study of 134 cases of circulating "lupus anticoagulant" (LA) observed between 1975 and 1985 in the haemostasis laboratory of Henri Mondor Hospital is reported. In 66 p. 100 of cases the circulating anticoagulant was discovered fortuitously and auto-immune diseases were associated with the inhibitor only in 25 p. 100 of cases (mainly systemic lupus erythematosus). Thrombosis (venous, arterial or both) was found in 26 p. 100 of cases when auto-immune diseases were present and in 13.5 p. 100 of cases in the absence of these diseases. Spontaneous abortion was observed in 42 p. 100 of the women when the anticoagulant occurred in the course of an auto-immune disease and in 4 p. 100 when another or no underlying disease were identified. False-positive VDRL and thrombocytopenia were found associated with LA mainly in auto immune diseases. Anticardiolipin antibodies were positive in 37 p. 100 of 32 patients without any difference between auto-immune diseases and other pathologies. These results show that the "lupus anticoagulant" is a frequent coagulation abnormality even when auto-immune diseases are absent. The differences observed between auto-immune diseases and other pathologies in respect to clinical manifestations and biological findings associated with lupus anticoagulant suggest that the so-called "lupus anticoagulant" represents a group of antibodies with probably different specificities but which act in a similar manner in "in vitro" coagulation tests.