We evaluated the usefulness of serum progesterone (P) determinations in differentiating between ectopic pregnancy (EP), normal intrauterine pregnancy (IUP), and abnormal IUP. Values were obtained from 233 samples from 54 patients with IUP, 100 samples from 26 patients with abnormal IUP, and 125 samples from 46 patients with EP. Although mean values from all three groups were significantly different, we could not detect a single value that readily predicted both the presence and absence of EP. Only 2% of patients with EP (excluding those having undergone ovulation induction) had a P greater than 20 ng/mL and only 2% of patients with IUP had a value less than 10 ng/mL. Thirty-one percent of IUP, 23% of abnormal IUP, and 52% of EP had values that fell between 10 and 20 ng/mL, which limits the clinical usefulness of this test.