INTRODUCTION: The aim of this study was to investigate the effects of imatinib mesylate on platelet aggregation and adenosine triphosphate (ATP) release in chronic myeloid leukemia patients. METHODS: Platelet aggregation and ATP release induced by 5.0 mM adenosine diphosphate, 0.5 mM arachidonic acid, 1.0 mg/ mL ristocetin, and 2 μg/mL collagen were studied by whole blood platelet lumi-aggregometer in 20 newly diagnosed chronic myeloid leukemia patients before and after imatinib mesylate treatment. RESULTS: At the time of diagnosis, 17/20 patients had abnormal platelet aggregation results; 8 (40%) had hypoactivity, 6 (30%) had hyperactivity, and 3 (15%) had mixed hypo- and hyperactivity. Repeat platelet aggregation studies were performed after a mean of 19 months (min: 5 months-max: 35 months) in all patients who received imatinib mesylate during this period. After therapy, 18/20 (90%) patients had abnormal laboratory results; 12 (60%) had hypoactive platelets, 4 (20%) had mixed hypo- and hyperactive platelets, and 2 (10%) had hyperactive platelets. Three of the 8 patients with initial hypoactivity remained hypoactive, while 2 developed a mixed picture, 2 became hyperactive, and 1 normalized. Of the 6 patients with initial hyperactivity, 4 became hypoactive and 2 developed a mixed pattern. All of the 3 patients with initial hypo- and hyperactivity became hypoactive. Finally, 2 of the 3 patients with initial normal platelets became hypoactive while 1 remained normal. There was a significant decrease in ristocetin-induced platelet aggregation after therapy (p<0.001), while platelet aggregation and secretion induced by other agonists showed no difference after treatment (p>0.05). DISCUSSION AND CONCLUSION: These findings indicate that a significant proportion of chronic myeloid leukemia patients have different patterns of platelet function abnormalities and imatinib mesylate has no effect on these abnormalities, with a significant impairment in ristocetin-induced platelet aggregation.