A prospective study was conducted to test the hypothesis that the absence of an intrauterine gestational sac when the serum level of human chorionic gonadotropin (hCG) is above 6500 mIU/mL is indicative of ectopic pregnancy. A total of 383 patients who were clinically suspected to have ectopic pregnancies had pelvic ultrasound examinations with serum hCG determinations on the day of the scan. There were 217 (57%) intrauterine gestations, 104 (27%) ectopic pregnancies, and 62 (16%) spontaneous abortions. Forty-one percent of patients had an hCG level above 6500 mIU/mL. The absence of an intrauterine gestational sac at an hCG concentration above this level had a sensitivity of 100%, a specificity of 96%, a positive predictive value of 86%, a negative predictive value of 100%, and was 98% efficient, based on a 19.4% prevalence of ectopic pregnancies among this group.