The doubling time (DT) of human chorionic gonadotropin (hCG) in serum was investigated retrospectively using serial serum hCG values that had been obtained from asymptomatic pregnant women with a prior history of infertility. The DT of hCG did not increase significantly as pregnancy advanced during the period in gestation when the serum hCG concentration was less than 10,000 mIU/mL (International Reference Preparation). Serum hCG concentrations increased subnormally in 2 of 60 women with normal intrauterine pregnancies, 5 of 8 women with asymptomatic ectopic pregnancies (EPs), and in 2 of 8 asymptomatic women who subsequently aborted their pregnancies. Neither the sensitivity nor the specificity of serial hCG testing for EP was enhanced by adopting different test criteria at different serum hCG concentrations.