Antithrombin-III (AT-III) and factor VIII coagulant (F VIII:C) and antigenic (F VIII:RA) activities have been studied in nine conservatively treated and 26 dialysed uraemic patients. AT-III levels were not significantly different from those of controls in either group. Among dialysed patients, those who had experienced thrombotic occlusions of the vascular accesses could not be distinguished from the remaining patients on the basis of their AT-III levels. Both F VIII:C and F VIII:RA were slightly higher than in controls in conservatively treated patients, but significantly higher in haemodialysed patients, especially in those who had never experienced thrombotic complications of the vascular accesses. No acute changes were observed in either the AT-III or F VIII:C/F VIII:RA ratio in five patients given heparin therapy during a dialytic session or in the interdialytic period. Thus repeated intermittent heparin treatment does not induce a hypercoagulable state in haemodialysed patients.