We retrospectively reviewed ultrasound findings in 539 infertility patients to evaluate the incidence of small gestational sac syndrome and its association with abortion, karyotype of the abortus, and known abortion factors. Small gestational sac syndrome (gestational sac diameter minus crown-rump length less than 5 mm) occurred in 1.9% of pregnancies scanned with vaginal ultrasound 37-65 days after the first day of the last menstrual period. The rate of fetal death was 80.0% when the gestational sac diameter-crown-rump length difference was less than 5 mm, 26.5% when the difference was 5-7.9 mm (P less than .002 compared with less than 5 mm), and 10.6% when the difference was 8 mm or more (P less than .0001 compared with less than 5 mm). Karyotypes were normal in all eight fetuses when the gestational sac diameter-crown-rump length difference was less than 5 mm (P less than .03). Small gestational sac syndrome was present before fetal death in 10.7% of all cases, and in 24% in which the karyotype was normal. Of the factors analyzed--maternal age, previous abortions, low hCG or progesterone levels, and use of ovulation induction medications--only maternal age was significantly different (P = .011) in patients with small sac syndrome. We conclude that small gestational sac syndrome is an infrequent but important complication of early pregnancy, which occurs more often in karyotypically normal than in abnormal fetuses.