Treatment with thalidomide is associated with vascular thrombosis. The effect of thalidomide on platelet activation is unclear, although the use of aspirin is justified for thromboprophylaxis. A study on platelet activation markers was done among multiple myeloma patients receiving thalidomide therapy with warfarin as thromboprophylaxis. Strict criteria and procedure were set to avoid misinterpretation of platelet activation other than due to the thalidomide's effect. Blood specimen pre and post thalidomide therapy were used for flow cytometric analysis. Platelet surface P-selectin, CD62P expression and PAC-1 (antibody that recognizes conformational change of the GPIIb/IIIa complex) were examined by using three-colour flowcytometer. Increased expression marker for PAC-1 was observed after 4 weeks of thalidomide treatment (P