The purpose of this study is to describe a phenomenon of bidirectional flow, "bicolor portal vein" (BPV), within the right anterior branch of the portal vein (RAPV), with color Doppler imaging (CDI). We prospectively studied with CDI the intrahepatic portal vein and its branches in 316 consecutive patients in search of areas of nonlaminar flow within the RAPV. Forty patients were excluded from the study due to varied conditions that precluded an accurate CDI study. A prospective design allowed us to record the frequency of appearance, systematically measure a set of parameters in each patient, and establish different groups for comparison. We found an area of BPV within the RAPV in six patients. Five of them had a history of liver disease (8.2% of the patients with known liver disease). The BPV was consistently portrayed as a well-defined blue oval-shaped area of flow reversal attached to the medial side of the RAPV immediately before its bifurcation, suggesting a rotary motion of blood. All six patients presented a transverse diameter of the RAPV significantly larger than the rest of the patients (p < 0.05). Furthermore, blood flow measurements within the RAPV of the six patients with BPV were found to be significantly higher when compared with the rest of the patients (< 0.05). Knowledge of the possible presence of a BPV within the RAPV can prevent diagnostic confusion with reversal of flow, a truly pathologic condition related to liver disease.