Leucocytosis (leucocyte count >15 x 10(9)/l) was recently associated with thrombosis in polycythaemia vera (PV). This study sought the prognostic relevance of leucocytosis for survival and leukaemic or fibrotic transformation. Amongst 459 patients with PV seen at our institution in recent years (median age, 60 years; 56% males), 146 deaths and 88 leukaemic (n = 34) or fibrotic (n = 54) transformations were documented. Arterial or venous thrombosis occurred in 14% and 9% of patients at diagnosis and in 25% and 15% during follow-up, respectively. Multivariate analysis identified the advanced age (P < 0.0001), leucocytosis (leucocyte count >/=15 x 10(9)/l; P = 0.0006) and arterial thrombosis at diagnosis (P = 0.01) as independent predictors of inferior survival. In the absence of the first two risk factors, median survival was projected at 272 months as opposed to 108 months in the presence of both risk factors (P < 0.0001). Leucocytosis was also identified as an independent predictor of both leukaemic transformation and venous thrombosis during follow-up. Time-to-event analysis did not disclose a significant association between single or multiple cytotoxic drug exposure and either leukaemic or fibrotic transformation. The current study highlighted the prognostic relevance of leucocytosis on various aspects of the disease in PV.