Serial serum hCG levels were measured in 50 patients with a tubal ectopic pregnancy and 50 patients with spontaneous miscarriage of an intrauterine pregnancy. Serum samples were obtained at intervals of 3–5 d and more frequently if clinically indicated. The final diagnosis was confirmed by laparoscopy and/or dilatation and curettage. Data were analyzed using a linear regression model. Initial hCG concentrations ranged from 91 to 3,050 mIU/mL. Eighty percent of ectopic pregnancies and 35% of miscarriages were associated with rising hCG concentrations and no significant differences were noted in daily increments of hCG in the two groups (210 ± 30 mIU/mL/day for ectopic pregnancies versus 311 ± 55 mIU/mL/d for miscarriages). Twenty percent of ectopic pregnancies and 65% of miscarriages had falling hCG concentrations and significant differences were noted in the daily decrements of hCG for EP and AB (270 ± 52 mIU/mL/day for ectopic pregnancies versus 578 ± 28 mIU/mL/d for miscarriages (P≤ 0.05).