A group of 590 women who, 4 to 5 weeks after their last menstrual period, were confirmed to be pregnant, as measured by the human chorionic gonadotropin (hCG) by radioreceptor assay. Nine of these women had serum hCG levels approximately twofold higher than the others and were suspected of having twin pregnancy. When these women were tested at 12 weeks of gestation, pelvic sonography confirmed twin pregnancies in all the nine cases. Serum hCG levels thus provide a simple, rapid, and easy method to detect twin pregnancy.