We studied changes in the expression of P-selectin on the blood platelet plasma membrane. The aim of the study was to determine the influence of renal carcinoma on P-selectin expression associated with changes in platelet morphology. Venous blood was collected from 30 patients with renal carcinoma and from 24 control subjects for cytometric analysis and to evaluate platelet morphology. P-selectin being the CD62P receptor on blood platelets was marked by anti-CD61/62P MoAb, and the results were presented as the percentage of CD62P-positive cells. Changes in the expression of the CD62P on the platelet plasma membrane during activation were investigated by flow cytometry in a comparative study of in vivo activation and in vitro platelet reactivity. Platelet activation reflected by P-selectin expression was higher in the group of patients (4.45 +/-1.96), compared to control (2.48 +/-1.66) (p < 0.05). However, adenosine diphosphate [ADP] -stimulated platelet reactivity in renal cancer patients increased only by 0.24% (p > 0.05), while following activation by thrombin by 0.54% (p < 0.05). Moreover, a higher (4.72 +/-2.02), statistically significant percentage of platelets with P-selectin expression was found in patients with disseminated neoplastic changes in renal parenchyma, compared to patients with a single localized neoplastic lesion (4.17 +/-1.89) (p < 0.05). A statistically significant difference was noted in the platelet count and anisocytosis in renal cancer patients. Renal cancer enhances P-selectin expression. It is due to the presence of intensified thrombinogenesis and other platelet agonists in the blood.