Chorionic gonadotrophin (hCG) was measured by a rapid and sensitive radioimmunoassay in 100 female patients with lower abdominal pain attending an emergency department. Of the 24 subjects with a positive result, 22 had evidence of pregnancy (ectopic or uterine). Of the 76 patients with negative results, 3 had some evidence of pregnancy. This test may be of considerable practical value in the differential diagnosis of acute abdominal emergencies in women of reproductive age.