OBJECTIVE To improve the accuracy and speed of diagnosis of an ectopic tubal pregnancy by means of blood flow analysis in the tubal arteries. We hypothesized that invasion of the trophoblast increases blood flow in the tubal artery involved in ectopic pregnancy. METHODS In 394 patients, using an endovaginal triplex color Doppler ultrasonography system, we performed qualitative blood flow analysis in the tubal arteries on both sides. The percentage of the between-side difference in tubal blood flow was calculated. RESULTS There was an increase in tubal blood flow on the ectopic pregnancy side, and the mean between-side difference in tubal blood flow was 20.45% in the ectopic pregnancy group. In the control groups, the between-side difference was 2.95% (t = 21.5, P < .00001). Using a cutoff point of 8% for the percentage of the between-side difference in tubal blood flow, the method had a sensitivity of 85% and a specificity of 96% for diagnosing an ectopic pregnancy. The percentage of the between-side difference in tubal blood flow was independent of gestational age (Pearson correlation coefficient 0.081). CONCLUSIONS The advantages of this new method for diagnosing tubal pregnancy are early detection, noninvasivity, and immediate results.