We report the case of a 27 year old woman, with no vascular risk factors other than moderate smoking, admitted for a first acute anterior myocardial infarction. Emergency coronarography detected an isolated thrombosis, localised at the level of the middle section of the anterior inter-ventricular artery in an otherwise normal coronary network. Angioplasty of the artery with direct stenting was performed successfully and the follow up was uncomplicated. The diagnosis of primary antiphospholipid syndrome was made on the association of a raised level of anti-cardiolipin antibodies and the absence of elements in favour of an associated connective tissue disorder.