Summary A number of growth factors are involved in clonal haematopoietic expansion and their clinical significance in patients with chronic myeloproliferative diseases requires further evaluation. Using enzyme‐linked immunosorbent assays, we analysed serum levels of interleukin (IL)‐1a, IL‐1b, IL‐2, IL‐6, the soluble IL‐2 receptor alpha (sIL‐2Ra), and thrombopoietin (TPO), in 25 individuals with myelofibrosis with myeloid metaplasia (MMM), 40 with essential thrombocythaemia (ET), eight with polycythaemia vera (PV), 10 patients with chronic myeloid leukaemia (CML) and 27 normal controls. These were correlated with clinicopathological characteristics including overall survival, and histopathological bone marrow features, including angiogenesis. The serum derived from patients with MMM, ET, PV and CML contained significantly higher IL‐2 and sIL‐2Ra than healthy subjects, while IL‐6 levels were higher only in MMM and CML than controls. IL‐2, sIL‐2Ra and IL‐6 levels were raised during the transformation phase of CML, during progression of MMM to AML, and ET and PV to myelofibrosis (P < 0·001). There was a positive correlation between IL‐2, sIL‐2Ra, IL‐6 and angiogenesis in bone marrow samples. Cytokines may be useful markers for predicting clinical evolution, reflecting increased angiogenesis. This requires further evaluation to guide diagnostic and therapeutic options.