Pentosan polysulfate may induce severe thrombocytopenia by an immunoallergic mechanism similar to that observed with heparin. We report five cases of serious arterial and venous thrombosis associated with pentosan polysulfate-induced thrombocytopenia: --two cases of fatal circumferential myocardial infarction due to disseminated thrombosis in patients whose coronary vessels were free from atheroma at angiography. Such cases have not previously been published. They seem to represent an unusual and dangerous manifestation of immunoallergic thrombocytopenia complicated by diffuse intravascular coagulation and induced by pentosan polysulfate; --two cases of myocardial infarction with favourable outcome; one of them was associated with hemiplegia with aphasia; --one case of extensive phlebitis of the inferior vena cava. The diagnosis was suspected on the ground that the patients had previously been treated with intramuscular pentosan polysulfate for 8 to 10 days and had less than 100,000 platelets per mm3; it was confirmed by in vitro platelet aggregation tests. Cross-allergy with heparin was present in all cases. Out of 18 published cases of pentosan polysulfate-induced thrombocytopenia, 15 were revealed by thrombosis of a coronary, cerebral or peripheral artery (80 p. 100) or of a vein (20 p. 100). Arterial thrombosis carries a 50 p. 100 risk of death, and for this reason pentosan polysulfate should be subjected to the same rules of prescription and monitoring as heparin.