OBJECTIVE To evaluate the significance of beta-HCG levels in peritoneal fluid and serum in the diagnosis of ectopic pregnancy. STUDY DESIGN SETTING: Obstetrics and Gynecology department of a regional general hospital. METHODS Sixty-two women who presented with a differential diagnosis of ectopic pregnancy vs. nonviable intrauterine pregnancy. METHODS All patients underwent D & C and culdocentesis. beta-HCG was measured in simultaneously obtained peritoneal fluid (PF) and serum (S), and the PF/S ratio calculated. RESULTS Twenty-three patients had an ectopic pregnancy (Group I). All 23 had higher beta-HCG concentrations in the PF than in the serum, with a mean PF/S ratio of 19.1 +/- 16.9. Twenty-four patients had an intrauterine pregnancy (Group II). The beta-HCG levels in the PF and serum were similar (mean PF/S 1.1 +/- 0.2). The difference in PF/S ratio between groups I and II was statistically significant (p < 0.001). CONCLUSIONS The measurement of beta-HCG in peritoneal fluid and serum is a useful diagnostic tool in differentiating ectopic from intrauterine pregnancy.