The clinical and laboratory experience with the lupus anticoagulant was reviewed in 37 patients. The anticoagulant is thought to act by blocking the activation of prothrombin by the prothrombin activator complex of factors Xa, V, and phospholipid. Although the anticoagulant has been principally associated with diseases of immune origin, 14 of the present patients had disorders not thought to be immune in nature. Eighteen patients underwent twenty-one operative procedures with only a single episode of excessive bleeding. In the authors' experience, the lupus anticoagulant is a rare cause of bleeding.Circulating anticoagulants directed against specific coagulation factors (XII, X I , I X , a n d VIII) have been sporadically reported in patients with systemic lupus erythematosus (SLE) (1-5). However the most common abnormality o f t h e coagulation system in SLE is the so-called lupus anticoagulant ( L A ) , which appears t o block the activation o f prothrombin by the prothrombin activator complex o f factors Xa, V, a n d phospholipid (6,7). T h e L A is characterized by a prolonged activated partial thromboplastin time ( P T T ) a n d by a mild to moderately prolonged prothrombin time (PT). T h e prolongation of t h e PT is greatly exaggerated when a dilute tissue thromboplastin reagent is utilized, a n d this feature forms the basis of a simple a n d specific test for t h e LA-the dilute tissue thromboplastin assay (7).During t h e past 4 years the authors have tested for t h e From