A pilot study was designed to analyze the vascular density of peritoneal endometriosis in 3 groups of lesions (red, black, and white) in 23 patients with peritoneal endometriosis who underwent laparoscopic surgery using the narrow-band imaging system and vascular analysis software. In the peritoneum, 21 red lesions were present in 10 patients, 12 black lesions were present in 9 patients, 12 white lesions were present in 8 patients, and 2 types of lesion were concomitantly present in 4 patients. Median vascular density of red, black, and white lesions under conventional light was 60.3%, 62.3%, and 60.6%, respectively, and under narrow-band light was 64.4%, 61.5%, and 62.0%, respectively, showing no significant differences among the lesions under either conventional or narrow-band light (p=.71 and p=.84, respectively). The median difference in vascular density under narrow-band and conventional light was not significantly different in black lesions (0.8%) or white lesions (1.0%); however, a difference of 4.5% was noted for red lesions (p <.001). We conclude that red lesions are indicative of early-stage endometriosis with angiogenesis. Use of the narrow-band system and vascular analysis software can enable accurate, objective, and reproducible evaluation of vascular density.