Hemagglutination assay of human chorionic gonadotropin (hCG) in the urine of patients suspected of having ectopic pregnancies has proved to be a highly sensitive method of detecting the condition. Moreover, the technique is simple and inexpensive. Hemagglutination assays were used in 167 patients with a diagnosis of ectopic pregnancy and in screening 415 patients in whom there was a possibility of ectopic pregnancy. In the former group, hCG titers in urine of more than 500 IU/liter were detected in 136 patients: among the remaining 31, the pregnancies were clinically old and resolving in 22. Low hCG titers were associated with a significantly shorter period of amenorrhea and a protracted clinical course. In the second group, a false-positive rate of 1.7% occurred when the sensitivity of the assay was limited. Increasing the sensitivity of the assay to avoid false-negative results, though also increasing the false-positive rate, would help to reduce the number of cases in which more complex isotope assays are required.