OBJECTIVE: To evaluate the degree of fibrosis in the stroma of peritoneal endometriosis, according to the macroscopic appearance. METHODS: Red and black lesions of peritoneal endometriosis were collected and type-I collagen was immunohistochemically identified. The percentage of the stained area in the stroma was measured using a computerized image analysis system. RESULTS: There was a significant difference in the mean percentage of the total stained area between red and black lesions. CONCLUSION: The amount of type-I collagen is significantly higher in the stroma of black peritoneal lesions than in red peritoneal lesions.