VEGF is a potent angiogenic regulator implicated in increased angiogenesis, which is characteristic of CML. VEGF interacts with its two tyrosine kinase (TK) receptors, Flt-1 (VEGFR-1) and KDR (Flk-1/VEGFR-2). CML therapy with imatinib mesylate, which targets BCR-ABL TK activity, induces haematological remission in 95% and complete cytogenetic remission (CCR)/ major response (MR) in 60% of cases. We describe the serial evaluation of patient bone marrow (BM) trephines and the analysis of the impact of imatinib on VEGF in all stages of CML during therapy. Consecutive CML (n=38) patients (25 males, 13 females, median age of 56 years (19 to 81) were sequentially analysed during the course of imatinib therapy. Chronic phase (CP) n=24, accelerated phase (AP) n=11 and blast crisis (BC) n=3 prior to imatinib therapy. BM examination was performed at diagnosis, at 1 month, then 3 monthly following imatinib therapy. Immunohistochemical analysis determined expression of VEGF, VEGFR1/Flt-1 or VEGFR2/KDR. Results were correlated with cytogenetic response. In normal BM cellular VEGF and its receptors were expressed in megakaryocytes and macrophages, but rarely in myeloid cells. VEGF was not expressed in erythroblasts, lymphocytes or plasma cells. A weak VEGFR1 signal was observed in monocytes/histiocytes and VEGFR2 in histiocytes only. VEGF expression in CML was detected as a diffuse cytoplasmic pattern in myeloid and monocyte precursors, classified as 4+ (very intense staining), 3+ (strong), 2+ (moderate), 1+ (weak), or 0 (completely negative) throughout. The intensity of staining varied and did not correlate with the stage of disease, but VEGF was strongly expressed in the megakaryocytes. Both VEGFR1/Flt-1 and VEGFR2/KDR were expressed in monocytic/myeloid population and in megakaryocytes. The staining intensity varied from borderline detectable to a very strong staining intensity. 14/38 (37%) of patients showed a reduction in VEGF expression, which was particularly marked with respect to VEGFR2 receptor staining. Of these, 10/14 patients (71%) achieved CCR, one MR (major response) and 3 minimal/NR (no response). All patients in this group had a reduction in BM cellularity, median 30% (15–70%). No change in VEGF receptor expression was detected in 10/38 (26%). Only 1 patient achieved CCR, 3 MR, with a minor/NR in the rest (60%), and a median BM cellularity of 30% (10–100%). Increasing VEGF expression was observed in 14/38 patients (37%). Here 6/14 (43%) patients showed NR (2 minor/minimal responses) with CCR in only 4 of the evaluable patients. The median BM cellularity was 25% (10–95%). 3/14 patients went on to develop progressive disease In summary, reduced expression of VEGF and its receptors, particularly KDR, can predict a favourable response to imatinib and correlates with a reduced BM cellularity and cytogenetic response. In patients with an increase or no change in VEGF expression there is a greater incidence of a poor cytogenetic response and a higher tendency to relapse despite a reduction in BM trephine cellularity, but a longer follow-up may be necessary.