Plasma antithrombin III (AT III) levels in hemodialysis patients, low prior to dialysis, improved after dialysis (p < 0.01). The platelet counts before and after dialysis, did not change significantly. In peritoneal dialysis patients the AT III levels, which were normal before dialysis, increased significantly after dialysis (p < 0.01). The platelet counts before and after peritoneal dialysis also improved (p < 0.005). No correlation was found between AT III levels and platelet counts. Although platelet damage has a contributory role in increasing AT III levels during hemodialysis, the data on peritoneal dialysis suggest that there may be other factors affecting platelets and AT III during dialysis.