Thirteen cases of heterotopic pregnancy were diagnosed among 1171 pregnancies established in Denmark after in-vitro fertilization and embryo transfer (IVF-ET). Thus the frequency of heterotopic pregnancy was 13/1171 or 1.1%. In five cases the diagnosis of heterotopic pregnancy was made by ultrasound at 6-9 weeks of gestation; three of these patients were asymptomatic, while two patients presented with abdominal pain and vaginal bleeding. All these patients had an unruptured ectopic pregnancy. Eight cases were diagnosed at the time of surgery; all these patients presented with abdominal pain. Only two of the 13 patients had vaginal bleeding. In nine of the 13 cases the intra-uterine pregnancy resulted in term delivery, while one pregnancy is ongoing. In pregnancies following IVF-ET, this diagnosis should particularly be considered in cases with abdominal pain; vaginal bleeding may be absent. Ultrasound examination may lead to early diagnosis even in asymptomatic cases. In most cases, removal of the ectopic gestation will allow the intrauterine pregnancy to proceed to term.