Hereditary factor XI deficiency may remain undiagnosed until severe bleeding is observed after an operation or trauma. Two such cases were encountered and, therefore, a regular screening test for coagulation disorders among urological patients was initiated. During 2 years (1975 and 1976) 10 additional cases of factor XI deficiency were detected: 5 severe and 5 partial. All patients were Ashkenazic Jews of Eastern European origin. The 5 patients with severe factor XI deficiency underwent an operation without any complications. They were transfused with 5 to 20 ml/kg/day of fresh frozen plasma from the day before the operation until 10 to 14 days postoperatively. Of the 5 patients with partial factor XI deficiency 4 underwent an operation. In 1 of 3 patients who did not receive transfusions postoperative bleeding was observed. A minimal level of 0.3 U/ml (30 per cent) factor XI was found necessary to ensure good hemostasis during and after an operation. In view of a recent finding of relatively high gene frequency of factor XI deficiency in Ashkenazic Jews it seems warranted to do a partial thromboplastin time in such patients who need an operation.