Platelet aggregation in whole blood (WB) and in platelet rich plasma (PRP) was studied in 18 consecutive patients affected by idiopathic myelofibrosis (IM). On the basis of WB studies, 22% of patients were classified as normo-aggregating and 22% as hypo-aggregating, while 55% had spontaneous platelet aggregation (SPA). SPA was observed also when platelets from patients were stirred in the presence of normal erythrocytes, while it never occurred when normal platelets were stirred in the presence of red cells from patients. PRP studies revealed that 33% and 66% of patients were, respectively, hypo- and normo-aggregating. The most frequent abnormality in PRP was represented by defective or absent aggregation response to epinephrine.