In polycythemia vera, gender has recently been shown to influence the JAK2(V617F) allele burden, but its effect on the disease phenotype is unknown. This issue was investigated using the database of the European Collaboration on Low-dose Aspirin in Polycythemia Vera (ECLAP) Study. The ECLAP Study recruited 1,638 polycythemic subjects and followed for 2.7 ± 1.3 years. At study entry, men, compared to women, had a higher prevalence of myocardial infarction (11.3 vs. 5.8%; P < 0.0001) and peripheral arterial disease (6.1 vs. 2.9%; P < 0.05) while a history of venous thrombosis was more common in women (11.4 vs. 7.9%, P = 0.016). Among 234 venous thrombosis, there were 39 splanchnic vein thromboses (33 extra-hepatic portal vein thromboses and 6 Budd-Chiari syndromes). Most of these events occurred as an early disease presentation in young female subjects. Women, compared to men, had higher platelet counts (average value 430 ± 213 vs. 375 ± 201 × 10(9)/L; P < 0.0001) and lower hematocrits (0.46 ± 0.06 vs. 0.48 ± 0.06 l/l; P < 0.0001). Cholesterol plasma level, available in 995 subjects (61%), was lower in male patients (180.8 ± 43.1 vs. 196 ± 46.6 mg/dl; P < 0.0001). During follow-up there were 205 major thromboses confirming an high incidence of myocardial infarction in men although not statistically significant (1.2 vs. 0.6 cases per 100 person-years; P > 0.05). These data show several gender-related differences both in the thrombotic diathesis and in the prevalence of vascular risk factors of PV patients.