In this prospective study among 208 high-risk patients with suspected ectopic pregnancy, the diagnostic value of transvaginal sonography and serum human chorionic gonadotrophin (HCG) measurements were analysed in detail. The absence of an intra-uterine gestational sac obviously was the most constant sonographic finding among patients with ectopic pregnancy (n = 89), with a very high sensitivity (0.99) but a low specificity (0.41). The application of different HCG cut-off levels improved specificity to 1.00 for values exceeding 4500 IU/l. Clinical utility obviously decreased, as many patients presented with HCG values well below this level. The additional effect of adnexal findings was analysed. Sonographic identification of an ectopic pregnancy was very specific (0.99) but had low sensitivity (0.56) because many ectopics were not detected. The additional effect of HCG values on these results was minor. The low HCG cut-off levels advocated in recent studies are questioned by the results of our analysis: whereas the combined use of sonography and HCG measurements is shown to be of great benefit, the limitations are also documented, underlining the need for re-evaluation at intervals of patients with low HCG values. The question of which cut-off level should be used in practice, however, hinges on a difficult choice between a certain specificity and clinical utility.