Previous reports have indicated that an intrauterine gestational sac is not usually detected when the maternal serum human chorionic gonadotropin (HCG) is less than 6000 mIU/ml. In order to evaluate this observation, maternal serum HCG concentrations were correlated with sonographic uterine findings in 49 patients with normal early intrauterine pregnancies. Of 37 patients in whom a gestational sac was seen, simultaneous HCG levels were 1800 mIU/ml or more in 36 cases and 357 mlU/ml in one case. In 12 cases in which a gestational sac was not detected, the serum HCG levels were 1400 mIU/ml or less. Thirteen patients had HCG levels of less than 6000 mlU/ml. A linear relation was found between gestational sac size and the exponential rise of HCG levels in early pregnancy (p less than 0.001). Of the 13 patients in whom HCG levels were less than 6000 mIU/ml, the gestational sac measured 10 mm or less in each case. Currently, a gestational sac is always seen when the HCG level is greater than 1800 mIU/ml. Comparison of serum HCG levels with sonographic detection of the gestational sac seems to be a useful method of evaluating early pregnancy.