The aim of this study was to assess prospectively the prevalence of anticardiolipin antibodies in patients with peripheral arterial thrombosis. Fifty-nine patients with arterial thrombosis (38 men, 21 women), ages ranging from 24 to 80 years, mean age, 45.5 years, were studied. The control group included 100 volunteer blood donors. After confirmation of the thrombotic event, anticardiolipin antibodies were dosed by the immunoassay enzyme method and by dilution. In Group I these antibodies were present in 62.7% of the patients, of which 33.9% were borderline and 28.8% were positive. In the control group 6% of the patients were borderline and 1% was positive. The statistical analysis showed that the prevalence found was statistically significant (p < 0.001), even when the borderline titers were not considered positive. Patients with peripheral arterial thrombosis have a high prevalence of anticardiolipin antibodies.