Objectives: To determine the seropositivity of Chlamydia antibody in patients with ruptured ectopic pregnancy compared to normal pregnant women and the risk factors for ectopic pregnancy. Study Design: This was a prospective case-control study of 85 cases of ruptured ectopic pregnancy and 100 cases of second trimester on-going intrauterine pregnant controls presenting in Lagos State University Teaching Hospital (LASUTH) between September 2009 and March 2010. Study Site: This was at the gynaecological emergency room and antenatal clinic in the Department of Obstetrics and Gynaecology. Ethical approval was sought and granted by the ethics review committee of LASUTH. Study Participants: Patients presenting with ruptured ectopic pregnancy were recruited as cases while the controls were made up of those with uncomplicated second trimester intrauterine pregnancy. A semi-structured questionnaire containing socio-demographic and clinical characteristics was administered following informed consent. Five milliliters of venous blood was taken from each participant and tested for Lymphogranuloma Venerum (LGV) type 2 broadly reacting antigen of Chlamydia trachomatis. Data Analysis: Data gathered from the case notes and laboratories were imputed into the computer and analyzed using the statistical package Epi-Info 3.51, Atlanta, USA. Frequency tables were generated for continuous variables and chi-square analysis used to determine association between variables, with p values <0.05 considered statistically significant. Results: There were 91 cases of ectopic pregnancy among a total of 2468 deliveries giving an incidence of 3.68% or 1 in 27 deliveries. Factors which significantly contributed to increased incidence of ectopic pregnancy in this study were: level of education (p = 0.001), socio-economic status (p = 0.001), parity (p = 0.005), early age of sexual debut (p = 0.001), multiple sexual partners (p = 0.001), previous pelvic inflammatory disease (p = 0.003), previous induced abortion (p = 0.013) and previous postabortal/puerperal sepsis (p = 0.013). The seropositivity of Chlamydia IgG (62.4%) in the cases was significantly higher than that of 29% in the control (p < 0.0001). Conclusion: There was a high incidence of ectopic during the period of study and the seropositivity of Chlamydia IgG antibody was significantly higher amongst the cases. Risk factors identified were low level of education, low socio-economic status, low parity, early age of sexual debut, multiple sexual partners, previous history of pelvic inflammatory disease, previous induced abortion and previous postabortal/puerperal sepsis.