In a 31-year-old woman with a history of recurrent arterial thrombosis, both of whose pregnancies had resulted in intrauterine death at 23 and 24 weeks, a "lupus" anticoagulant was identified. The patient's IgG fraction, containing the lupus anticoagulant, reduced the release of prostacyclin (PGI2) from rat aorta rings or pregnant human myometrium. This inhibitory effect was abolished in the presence of arachidonic acid. The production of 6-keto-PGF1 alpha by cultured bovine endothelial cells was also decreased in the presence of the patient's IgG fraction. The plasma level of 6-keto-PGF1 alpha was reduced. An antibody in this patient may interfere with the production or release of PGI2 by the vessel wall, possibly by interfering with the availability of arachidonic acid. This mechanism could play a role in this patient's arterial disease and obstetric problems.