We conducted a study to compare the performance of serum and urine pregnancy tests in the evaluation of gynecologic patients presenting to our ED. The overall efficiency of the two tests was very similar: 99.5% for the serum test, and 97.6% for the urine test. In patients proven to have ectopic pregnancies, however, the serum test was positive in 100%; the urine test was positive in only 60%. The serum test misclassified (gave false-negative or false-positive results) in three of 607 patients (0.5%). The urine test misclassified 14 of 607 patients (2.3%). Moreover there were 18 inconclusive or invalid urine test results. Thus the urine test provided misinformation or no information in 32 of 607, or 5.3%, of the total study population.