This study demonstrates that a low haematocrit is the main determining factor of the prolonged bleeding time often encountered in uraemic haemodialysed patients. Thirty-three patients submitted to regular haemodialysis and having a platelet count greater than 100 X 10(9)/l were investigated with the following tests: simplate bleeding time, blood cell count, platelet aggregation induced by ADP, collagen and sodium arachidonate, arachidonate induced MDA synthesis, tests for detection of an acquired storage pool disease, and factor VIII complex level. The results were compared to two other groups; one of uraemic patients not yet subjected to haemodialysis and another of healthy volunteers. The results were basically identical in the two groups of uraemic patients. The only consistent abnormality was a 30-35% reduction in the platelet MDA synthesis in comparison with control subjects. There was a negative correlation between the log bleeding time and the haematocrit (r = 0.78, P less than 0.01). Fourteen uraemic patients having a prolonged bleeding time were submitted to a red cell transfusion programme and were investigated a second time under identical conditions. There was no change in any of the platelet function tests or in the factor VIII complex level, but the bleeding time was normalized when the post-transfusion haematocrit was over 26% (nine patients). This study emphasizes the role of anaemia in the pathogenesis of the prolonged bleeding time in uraemia and suggests that red cell transfusion can be a long-term efficient therapeutic measure to stop bleeding in these patients.