One hundred suspected ectopic pregnancies were assessed by ultrasound on the basis of the following criteria: (A) viable intrauterine fetus, intrauterine pregnancy is certain; (B) intrauterine double sac or eccentric ring, intrauterine pregnancy is probable; (C) empty uterus or central ring but no adnexal mass or cul-de-sac fluid, ectopic pregnancy is possible; (D) empty uterus or central ring and an adnexal mass or cul-de-sac fluid, ectopic pregnancy is probable; (E) viable ectopic fetus, ectopic pregnancy is certain. Serum human chorionic gonadotrophin (S-hCG) was detected by an immunofluorometric assay (sensitivity 0.2 i.u./l, cut-off level 10 i.u./l). All the 51 patients in groups A and B had an intrauterine pregnancy. Normal gestational sacs were found also at S-hCG levels of less than 3600 i.u./l, the lowest level being 894 i.u./l. Ectopic pregnancy was confirmed in 29 of the 30 women in groups D and E. In the 19 women categorized into group C serial hCG assay and repeated sonography diagnosed ectopic pregnancy in 12 and miscarriage of an intrauterine pregnancy in the other seven. Ectopic pregnancy was always found when no gestational sac was seen by sonography and the hCG level was greater than 1000 i.u./l.