A 52-year old woman with congenital cyclooxygenase deficiency who had moderate bleeding tendency and severe generalized arteriosclerosis is reported.Diagnosis based on the following data: secondary platelet aggregation induced by adrenaline, ADP, ristomycin, collagen, thrombin was absent; platelet completely failed to aggregate by arachidonic acid while aggregation induced by calcium ionophore A23187 was normal; no malondialdehyde formation could be detected in the platelet rich plasma /PRP/ in four different times /for exclu- sing any drug effects/. The abnormal adrenaline and ADP induced aggregation were not corrected when patient’s PRP was mixed in equal proportions with that of a normal subject ingesting aspirin.Although our patient had been free from severe thrombotic episodes, expressed signs of generalized arteriosclerosis could be detected.Until now only five cases of congenital deficiency of cyclo-oxygenase have been described /Malmsten et al., Weiss and Lages, Lagarde et al., Pareti et al./. However, in these patients no signs of arterial vascular diseases were mentioned.The special importance of this new case comes from the fact that life-long deficiency of cyclooxygenase enzyme could not protect from progressive vascular disease which might prove again that chronic intake of large doses of aspirin cannot prevent arterial disorder.