Antiprothrombin antibodies (aPT), a new serologic marker of antiphospholipid syndrome, were studied in 46 patients randomly selected from 73 with Sneddon's syndrome and 20 matched normal controls. aPT were elevated in 26 patients (57%) and were not found in any of the controls. The addition of aPT data increased the proportion of Sneddon's syndrome patients with at least one type of antiphospholipid syndrome marker from 65 to 78%. The finding that aPT are common in Sneddon's syndrome supports the hypothesis that Sneddon's syndrome is a form of antiphospholipid syndrome.