The Philadelphia chromosome-negative (Ph−) chronic myeloproliferative disorders (MPD) have an inherent tendency for transformation into acute myelogenous leukaemia (AML). The long-term rate of leukaemic transformation in unselected MPD patients was studied in well-defined MPD populations in Gothenburg, Sweden and the Côte d′Or area, Burgundy, France, respectively. Over a median observation time of 15 yr, 56 subjects (7%) out of a total of 795 patients with Ph− MPD transformed to AML. The yearly incidence of AML transformation was 0.38% in polycythaemia vera (PV), 0.37% in essential thrombocythaemia (ET) and 1.09% in idiopathic myelofibrosis (IMF). The incidence of AML development was significantly higher in IMF as compared with both PV and ET (P = 0.002 and P = 0.02, respectively). Six of the patients who developed AML had never been treated with cytoreductive agents and two had only been exposed to interferon. In IMF, the average time from diagnosis to AML transformation was 42 ± 33 months, which was significantly shorter than for both PV and ET (88 ± 56 and 76 ± 57 months; P = 0.0075 and P = 0.027, respectively). The time from diagnosis to AML transformation appears to be a continuous event as regards all three MPD entities. It was shown that 17 out of the 18 patients with PV who developed AML were females; this was true despite the fact that the male/female ratio for the whole PV group was 146/171 (0.85). As regards ET and IMF patients who transformed to AML, the gender ratio showed slight male predominance (1.33 and 1.13, respectively). The average survival time for the 56 MPD patients who developed AML was 4.6 ± 5.5 (range 0–28) months and did not differ with respect to the three subtypes of pre-AML MPD.