Background: Endometriosis constitutes a very important problem in gynaecological practice especially with respect to infertility and chronic pelvic pain. The relationship of endometriosis with ABO blood group system remains a controversial issue. Other risk factors like prolonged period of menstruation, personal habits and familial tendency have been implicated. Setting: Gynaecological clinic of the University College Hospital, Ibadan. Objectives: To study the prevalence of endometriosis among women undergoing diagnostic laparoscopy for chronic pelvic pain and infertility and its relationship with ABO blood group system and other risk factors. Methodology: Fifty five women being evaluated for chronic pelvic pain and infertility with laparoscopy had their blood samples taken for blood grouping. Their demographic data and history were taken. Physical and pelvic examinations were performed and relevant information filled in the questionnaire. Endometriosis was diagnosed using diagnostic out-patient laparoscopy under conscious sedation. Staging was done using revised classification of the American Society of Reproductive Medicine