We measured platelet function by standard aggregometric tests and by the PFA-100 (Dade Behring, Newark, DE) in samples from 55 patients with primary thrombocythemia (PT) and 26 healthy volunteers. Platelet function was evaluated in platelet-rich plasma by aggregation tests. PFA-100 studies (closure time) were performed in citrated whole blood using collagen-adenosine diphosphate (ADP) and collagen-epinephrine cartridges. Plasma levels of von Willebrand factor (vWF) also were measured. The mean +/- SD closure time for patients vs volunteers for the collagen-epinephrine cartridge was prolonged (210.8 +/- 62.2 vs 118.1 +/- 19.6 seconds; P < .001); results were abnormal for 38 patients (69%). Results with the collagen-ADP cartridge also were abnormal in patients (134.3 +/- 58.4 seconds) vs volunteers (87.3 +/- 15.6 seconds; P < .001); closure time was prolonged in 23 patients (42%). A decreased response to epinephrine (38.4% +/- 34.2% vs 82.5% +/- 10.3%; P < .001), the main defect detected by platelet aggregation tests, affected 32 patients (58%). Platelet response to collagen also was abnormal (52.0% +/- 34.6% vs 86.0% +/- 10.1%; P < .01) but affected only 21 patients (38%). vWF levels for patients were normal. The results seem to confirm that platelet function in patients with PT is abnormal and show that platelet function can be assessed by an easy, reproducible, and sensitive method, the PFA-100. Closure time usually was prolonged; this feature could be applied in the diagnosis of PT.