The Coulter counter, model S Plus, Provides a platelet count and a mean platelet volume in all routine specimens of blood for cell count. The value of mean platelet volume in the prediction of the haemostatic potential of thrombocytopenic patients was investigated in 175 patients with haematological disorders who underwent 1473 blood counts over five months. Eighty-four haemorrhagic episodes were detected, most in thrombocytopenic patients. The mean platelet volume of patients with haemorrhagic tendency was significantly lower (5.52 +/- SD 0.7 fl) than that of patients without these tendencies (7.87 +/- SD 1.75 fl) (p less than 0.001). In cases of severe thrombocytopenia (less than 20 x 10(9)/1 platelets) haemorrhagic episodes were frequent; however, the frequency of bleeding was considerably lower in cases in which the mean platelet volume was higher than a suggested cut-off point of 6.4 fl. Discriminant analysis selected mean platelet volume as more important than platelet count for prediction of haemorrhagic state in severe thrombocytopenia. In view of the useful discrimination that mean platelet volume provides between thrombocytopenic patients who bleed and those who do not bleed, it may serve as a guide to predict the danger of haemorrhage and the need for prophylactic platelet transfusion.