Summary Microvessel density of benign, borderline and malignant ovarian tumours was studied immunohistochemically using antibodies to the endothelial cell markers CD31, CD34 and factor VIII-related antigen. Microvessel density was compared in tumours of different histological subtype, stage and patient outcome. CD31 -immunostained sections were examined and regions of high and average microvessel density were selected. Identical regions were located on CD34-and factor Vill-related antigen-immunostained serial sections and microvessel counts obtained and converted to vessels mm-2. CD31 and CD34 immunostaining revealed increased microvessel density in both the high and average vessel density regions of mucinous (222.4 ± 24.8; 79.9 + 8.5) compared with serous (105.4 + 20.7; 33.3 ± 6.8) and benign (84.4 + 19.4; 20.4 + 4.4) tumours (P < 0.001). CD31 and CD34 immunostaining also revealed increased microvessel density in early-stage mucinous tumours (234.6 ± 28.2; 87.8 + 9.2) compared with that observed in both early-(72.8 + 15; 12.9 ± 2.4) and late-(115.6 ± 26.5; 29.8 + 8.5) stage serous tumours (P < 0.001). No differences in microvessel density in samples from patients with differing outcomes were observed (P > 0.05). Reduced factor VIII-related antigen compared with CD31 and CD34 immunostaining was observed in both borderline and malignant mucinous and serous tumours (P < 0.02) but not in benign tumours (P > 0.05). Our results contradict the putative association between increased microvessel density and poor prognosis and suggest that the level and control of angiogenesis may differ between ovarian tumour types.