Microvessel density (MVD) has been studied in a number of neoplasias, and apparently, there is a relationship between angiogenesis and tumor progression, response to treatment, and outcome. In pituitary adenoma, the association between MVD and vascular endothelial growth factor (VEGF) with tumor behavior has been described, but correlation with other angiogenic factors such as fetal liver kinase 1 (Flk-1) or proliferative markers is unknown. We investigated MVD, VEGF, and its receptor Flk-1 expression in 60 human pituitary adenomas: 13 growth hormone cell adenomas, 7 prolactin cell adenomas, 5 corticotroph cell adenomas, 2 thyrotroph cell adenomas, and 33 nonfunctioning adenomas (30 gonadotroph cell adenomas and 3 null cell adenomas). We performed immunohistochemistry for CD34, Ki-67, VEGF, and Flk-1. To evaluate MVD, we used 2 methods: the number of vessels per square millimeter and the Chalkley method. Immunohistochemistry results were correlated, as well as with clinicopathologic factors. Adenomas with higher MVD were thyrotroph cell adenomas (299.9 +/- 87.5), and those with lower MVD were prolactin cell adenomas (168.6 +/- 63.3; P = .45, analysis of variance). We found a trend toward higher MVD in the adenomas of older patients (P = .142), but no difference was found regarding sex, extrasellar extension, or Ki-67 (P > .05). However, extrasellar extension was nearly significant when the Chalkley method score was high (P = .056). Low expression of VEGF was seen predominantly in prolactin cell adenomas, and high in nonfunctioning adenomas, or in cases of older patients (P < or = .032). Flk-1 score correlated with VEGF (P = .006). High expression was observed in nonfunctioning adenomas, cases presenting at older ages, and with extrasellar extension (P < or = .022). Our study shows that VEGF and Flk-1 are widely expressed in pituitary adenomas, predominantly in nonfunctioning adenomas and those presenting at older ages. Moreover, Flk-1 is associated with a more aggressive phenotype, and it may have potential therapeutic interest.