It has been shown that large platelets are hemostatically more active than the smaller ones. We therefore studied the relationship between the mean platelet volume and the percentage of micro- and mega-thrombocytes measured by a Coulter counter S plus II, and the bleeding tendency in 57 unselected patients with a platelet count below 50 X 10(9)/l. We found no significant differences for any of these parameters between patients without and those with mild or severe bleeding tendency. This also held true when patients with a possible platelet dysfunction or with coagulation abnormalities were excluded. We conclude that platelet volume analysis in unselected patients with severe thrombocytopenia is not helpful in the prediction of their risk of bleeding.