Ectopic pregnancy is not recognized in 5523% of all cases using state-of-the-art diagnostic procedures. In this study, blood flow in tubal arteries was measured using a transvaginal duplex Doppler ultrasonography system. A 3.5 MHz Doppler transducer coupled to a 5 MHz imaging transvaginal sector probe was used to depict flow characteristics in the tubal branch of the uterine artery. This was performed in 102 patients. There was a significant increase in the blood flow on the tubal gestation side (p < 0.0001; z = -4.08). This between-side difference was determined using qualitative frequency shift analysis. The mean reduction in the resistance index on the side with the ectopic pregnancy as compared to the contralateral side was 15.6%. These changes appear to be due to trophoblast invasion. Between-side differences showed no dependence on gestational age (between gestational weeks 4 and 12 postmenstruation). We compared these data with those from three control groups ('early viable intrauterine pregnancy', 'early intrauterine pregnancy failure', 'non-gravid state'). In all these control groups, the impedance to flow, expressed as 'resistance index', showed no significant between-side difference. All control groups had significantly higher mean resistance index values than the ectopic pregnancy side, but did not differ from the contralateral side of the ectopic pregnancy group. The advantages of this new method in diagnosing ectopic pregnancy are early detection, non-invasivity, and immediate results.