It has been recently shown that in vitro platelet aggregation is inhibited when platelet concentration in platelet-rich plasma (PRP) is "normalized" by the addition of platelet-poor plasma (PPP). In this study we tested the hypothesis that the large amount of PPP required to "normalize" PRP in patients with thrombocytosis may result in falsely defective platelet function. To this end, we evaluated platelet aggregation in PRP samples "normalized" with either PPP or buffer in 16 patients with high platelet counts induced by myeloproliferative disorders. Comparison with the results obtained in healthy subjects demonstrated that patients had reduced platelet responses to ADP or collagen in PRP/PPP samples, but normal responses in PRP/buffer. By contrast, the majority of patients had severely defective platelet response to epinephrine independently from the methodological approach. We suggest that the reduced in vitro platelet aggregation previously described in patients with myeloproliferative disorders and thrombocytosis partially derived from a laboratory artefact.