Vascular endothelial growth factor (VEGF) appears to be implicated in tumour angiogenesis. In the present study immunohistochemical expression of VEGF was evaluated in 34 oligodendrogliomas (13 grade II, 21 grade III [WHO]). VEGF immunoreactivity was found in 31 of 34 cases. Expression of VEGF was observed in endothelial cells and some vascular smooth muscle cells, but not in neoplastic oligodendrocytes. Vessel counts, percentages of VEGF-positive vessels and vessels with vascular endothelial proliferation were assessed. The degree of VEGF labelling and vascular-endothelial proliferation in each vessel were evaluated using a 3 degree intensity score. Expression of VEGF was higher in grade III than in grade II oligodendrogliomas as assessed by percentage of VEGF positive vessels (55.8 +/- 29.2% vs 17.0 +/- 19.0% [P < 0.001]) and by VEGF immunostaining intensity (1.90 +/- 0.60 vs 0.90 +/- 0.40 [P < 0.001]). VEGF expression did not correlate with vessel density. Intensity of VEGF expression correlated positively with that of vascular-endothelial proliferation in grade III tumours (r = +0.47 [P < 0.05]). The percentage of VEGF positive vessels showed some degree of positive correlation with the percentage of vessels showing vascular-endothelial proliferation (r = +408 [P < 0.10]). Within individual grade III tumours 67.5 +/- 29.6% of all vessels with vascular-endothelial proliferation were VEGF-positive and 31.0 +/- 20.5% of all VEGF-positive vessels showed no evidence of vascular-endothelial proliferation. We conclude that (i) expression of VEGF is observed in the vasculature of oligodendrogliomas; (ii) marked expression of VEGF is observed in grade III oligodendrogliomas; (iii) VEGF may be one of the interrelated causative stimuli acting in concert to induce vascular-endothelial proliferation.