The prognostic significance of microvessel quantitation in invasive breast carcinoma was analyzed in a study group that comprised 88 patients with axillary node-negative carcinoma and 32 patients with axillary node-positive carcinoma who had a minimum follow-up period of 9 years. Microvessels were identified by immunohistochemistry using antibodies to endothelial markers, including factor VIII-related antigen and blood group isoantigens (ABH). Factor VIII-related antigen staining provided more consistent results for microvessel quantitation than did staining for ABH isoantigens. The three most vascular areas within a tumor were selected, and the microvessels within a x200 microscopic field of each area were counted by two investigators simultaneously. Node-positive carcinomas demonstrated significantly higher microvessel counts than did node-negative carcinomas (mean +/- SD, 99 +/- 42 and 73 +/- 22, respectively; P less than .001). In node-negative carcinomas, tumors from patients who experienced distant recurrence had higher microvessel counts than did tumors from patients who were disease-free (84 +/- 19 and 70 +/- 22; P = .01). Similarly, in patients with node-positive carcinoma, microvessel counts were considerably higher in tumors from patients who experienced distant recurrence than in patients who did not, although the difference did not reach statistical significance (113 +/- 44 and 93 +/- 34, respectively). Among patients with node-negative carcinoma, those with a microvessel count of less than 84 had a recurrence rate of 20% compared with 57% in patients with counts greater than 84 (P = .003). Microvessel counts were independent of histologic parameters, ploidy status, and S-phase fraction but correlated with peritumoral vascular invasion. Both microvessel counts and vascular invasion were independent prognostic parameters by multivariate analysis. High vessel counts may represent increased tumor angiogenesis and are correlated with tumor aggressiveness. Microvessel quantitation may be an additional prognostic factor that, when used in conjunction with more established parameters, can help in appropriate patient management.