In a prospective study using a cutoff value of 140 pg/mL, serum estradiol 17-beta assay had a sensitivity of 83% and a specificity of 100% in differentiating ectopic pregnancy (6 patients) from normal pregnancy with threatened abortion proceeding to viability (7 patients). In differentiating threatened abortion from spontaneous abortion (9 patients), the estradiol assay had a sensitivity of 88.9% and a specificity of 100%. All but one of the patients with ectopic pregnancy had estradiol levels below the cutoff value of 140 pg/mL, as did all but one of the patients who had spontaneous abortion. All the patients who had threatened abortion that progressed to viability had values well above the cutoff level. The mean estradiol values for the viable pregnancy group were significantly different from those of the other two groups. These data suggest that, at the institution where this study was done, serum estradiol determinations may be of value in the differentiation of both ectopic pregnancy and spontaneous abortion from threatened abortion but appears to be of very limited usefulness in distinguishing ectopic pregnancy from spontaneous abortion. The validity of these conclusions is limited by the small number of subjects. Further studies comprising greater numbers of subjects are needed.