Circulating endothelial cells and their precursors are suggested by some authors to be novel markers of angiogenesis. The aim of the study was to measure circulating endothelial cells (CEC), circulating endothelial precursors (CEP) and activated endothelial cells (aCEC) and serum concentrations of VEGF (vascular endothelial growth factor) and bFGF (basic fibroblast growth factor), well-known proangiogenic factors in patients with haematological malignancies before and after chemotherapy. Measurements were carried out in 20 patients with acute leukemia, 21 with malignant lymphoma and with 20 with multiple myeloma. The number of CEC, CEP and aCEC was measured by means of 3-color flow cytometry and serum concentrations of VEGF and bFGF with ELISA. In patients with acute leukemias and lymphomas the number of CEC was significantly higher than in controls, and that high number correlated with worse prognosis in patients with lymphomas. The increased number of CEP at diagnosis in patients with acute leukemia and lymphoma correlated with worse prognosis. The number of aCEC was higher in leukemic and lymphoma groups. After chemotherapy the decrease in CEC and CEP numbers in patients with acute leukemia and lymphoma was observed. In patients with lymphoma the increased serum VEGF concentrations in comparison with healthy subjects were noted but in leukemic patients-lower concentrations of VEGF. The initial high concentrations of bFGF in all patients did not change after therapy and in patients with lymphoma correlated with worse prognosis. Results suggest that in patients with acute leukemias and lymphomas CEC and CEP may be the markers of malignant process correlating with clinical outcome. aCEC may have a similar role in both diseases. Also in patients with lymphoma VEGF may be a marker of disease activity. bFGF is connected with pathogenesis of acute leukemia, myeloma and lymphoma and in patients with lymphoma is a predictor of worse prognosis.