Angiogenesis has been shown to be related to tumour behaviour, prognosis and response to treatment in many different tumour types. The aim of this study was to examine the relationship between angiogenesis and tumour behaviour and response to treatment in pituitary adenomas. The microvessel density (MVD) of pituitary tumours was assessed by counting blood vessels labelled with 3 different endothelial markers using antibodies to CD31, factor eight-related antigen and biotinylated Ulex europaeus (agglutinin I UEAI). One hundred and forty-two surgically removed pituitary adenomas (46 GH secreting, 6 microprolactinomas, 19 macroprolactinomas, 18 ACTH secreting and 53 functionless tumours) were carefully characterized and assessed. There was a significant negative correlation between age and MVD of GH secreting tumours (R2= 33· 8, P= 0· 005). Age was not related to MVD in other tumour types. Pre-treatment hormone production by the adenomas was related to MVD in prolactinomas (P< 0· 05), but not in GH secreting tumours. Invasive prolactinomas were significantly more vascular than non-invasive tumours (P< 0· 05). Drug treatment with metyrapone or bromocriptine did not appear to influence tumour angiogenesis. Surgical cure was more likely in macroprolactinomas and in ACTH secreting tumours with lower MVD. These results show that factors related to angiogenesis are very important in determining a number of clinical features of pituitary tumours, in particular the invasiveness of macroprolactinomas, the effect of age in tumours secreting GH and the outcome of surgical treatment in macroprolactinomas and ACTH secreting tumours.