Shpilberg and Shahar correctly emphasize that the main limitation of our study is inherent in its retrospective design. Thus, any finding should be interpreted cautiously, and any conclusion considered as a working hypothesis should be verified prospectively.

Several hypotheses and open questions exist about the diagnosis of polycythemia vera, given the sparse or outdated data available [1]. Prospective studies of polycythemia vera whose results have been published are limited by small sample size and by discrepancies in inclusion criteria. Apart from the studies of the Polycythemia Vera Study Group [2] and the trial organized by the European Organization for Research on Treatment of Cancer [3], no other relatively large randomized, controlled trials can be found in the literature. Our retrospective study, although by far the largest in the literature, could not compensate for this lack of knowledge.

As clearly stated in our paper, our primary goal was to provide information for designing clinical trials. Thus, the incidence of thrombotic events in polycythemic patients estimated in the retrospective analysis was used to calculate the sample size for a double-blind, placebo-controlled clinical trial testing low-dose aspirin in patients with polycythemia vera. Our other main objective was to explore the feasibility of organizing an active Italian collaborative group for the study of polycythemia vera. Following this experience, a European Collaboration on Low-dose Aspirin in Polycythemia Vera was established and funded by the European Union BIOMED 2 program. This study is a randomized trial designed to assess the risk–benefit profile of low-dose aspirin in patients with polycythemia vera. Several European countries are involved in the study, which will enroll approximately 3000 patients to be followed for 3 to 4 years. Patient recruitment began in June 1996 and is expected to be completed by December 1998. Patients not randomly assigned to treatment will also be followed. We hope that this prospective study will be helpful in assessing the current natural history of the disease.