To evaluate angiogenesis in endometrial hyperplasia and stage I endometrial carcinoma, and to investigate the relationship between angiogenesis and tumor grade and depth of invasion. Three groups of patients were analyzed: control patients who underwent hysterectomy for benign conditions (n = 19), patients with endometrial hyperplasia (n = 24), and patients with stage I endometrial carcinoma (n = 34). All hysterectomy specimens were stained immunohistochemically for factor VIII-related antigen as a sensitive and specific marker for vascular endothelium. Areas close to the deepest myometrial invasion or those with the highest grade of endometrial hyperplasia and the highest angiogenic intensity were selected. Three fields (x 400) were selected for each slide, and the mean microvessel count was calculated. Statistical analysis included Mann-Whitney U test or Kruskal-Wallis analysis of variance and Dunn post hoc procedure. P < .05 was considered significant. A significant difference was found between the microvessel count of controls versus the group with complex endometrial hyperplasia (median 21, range 16-80, versus median 38, range 20-130; P < .05). Microvessel counts of complex endometrial hyperplasia were significantly higher than those of simple hyperplasia (median 25, range 16-42; P < .05) and significantly lower than counts of endometrial carcinoma (median 77.5, range 19-189; P < .05). Microvessel counts in complex hyperplasia were not significantly different than those of noninvasive stage I endometrial carcinoma (median 38, range 20-130, versus median 44, range 19-119; P = .5). In cases of stage I endometrial carcinoma, a higher number of microvessels was noted in specimens with myometrial invasion than in those without myometrial invasion (median 44, range 19-119, versus median 83, range 19-189; P < .01). A higher number of microvessels was noted in cases with grade 2 than in those with grade 1, stage I endometrial carcinoma (median 44, range 19-98, versus median 96, range 63-189; P < .001). Complex endometrial hyperplasia and endometrial carcinoma are angiogenic. Furthermore, in stage I endometrial carcinoma, greater depth of invasion and higher tumor grade are directly correlated with angiogenic intensity.