A 30 year old man admitted with a brain stem infarct presented with intellectual deterioration. A diagnosis of systemic lupus erythematosus (S.L.E.) was based on the presence of five A.R.A. criteria :photosensitivity, arthralgia, leukopenia and thrombopenia, false positive syphilitic serology, antinuclear factors on immunofluorescence. A lupus type circulating anticoagulant (L.C.A.) was no longer detected after corticotherapy. This appears to be a case of the "hematologic" form of S.L.E. in which the presence of L.C.A. predisposes towards thrombosis, not only of veins--which is typical--but also of arteries. These may be isolated and reveal the underlying disease as in the present case. The L.C.A. is an antibody possessing antiphospholipid specificity which explains the anticoagulant action in vitro and the thrombogenic effect in vivo. Some authors consider this to be sufficient justification for administration of anticoagulants and/or antiplatelet agents, but corticotherapy may be effective.