The system involving angiopoietin-2 (Ang-2) and its receptor, Tie-2, appears to play an important role not only in tumor angiogenesis, but also in the biology of haematological and non-haematological malignancies. In the present study we evaluated the serum levels of soluble Ang-2 (sAng-2) and soluble Tie-2 (sTie-2) in patients with haematological malignancies. Measurements were carried out in 15 patients with chronic myeloid leukaemia (CML), 25 with essential thrombocythemia (ET), 24 with multiple myeloma (MM) and six with monoclonal gammopathy of undetermined significance (MGUS). In addition, we correlated the levels of angiopoietins with known prognostic factors. sAng-2 and sTie-2 were quantified with enzyme-linked immunosorbent assay (ELISA). In patients with CML and MM the levels of sAng-2 were significantly higher (1686.53 +/- 936.41 pg/mL and 1917.82 +/- 1427 pg/mL, respectively) than in controls (n = 15; 996.096 +/- 414.65 pg/mL) (P < 0.01). In patients with MM sAng-2 levels were significantly increased with increasing stage of disease, from stage I to stage III (P < 0.03) and presented a trend of correlation with Beta2-microglobulin levels (r = 0.317) and grade of bone involvement. Furthermore, the levels of sAng-2 determined after 6 months of chemotherapy in CML patients were significantly lower than at diagnosis in the patients who achieved haematological remission. Circulating sTie-2 levels were increased in patients with ET (17.5 +/- 9.2 vs 9 +/- 3.5 ng/mL; P < 0.01) and in those with CML (16.29 +/- 8.7 ng/mL; P < 0.04). In conclusion, abnormal levels of sAng-2 and sTie-2 are present in some haematological malignancies. These markers may play a role in the pathophysiology of these conditions and their progression.