RATIONALE AND OBJECTIVES An in vitro perfused bovine liver model was used to evaluate the relationship between the sizes of radiofrequency ablation lesions and variation in portal venous blood flow. MATERIALS AND METHODS Fourteen bovine livers were perfused with autologous heparinized blood at 37°C and 40% to 50% oxygenation via the portal vein. Flow rates were adjusted from 10 to 50 mL/min/100 g tissue. A 480-kHz generator and a 3.0-cm monopolar internally cooled electrode were used to create 57 ablations. The long-axis diameter, short-axis diameter (SAD), and volume of each ablation zone were measured and calculated from the dissected livers. Correlations between SAD, long-axis diameter, and volume versus blood flow were assessed using linear regression analysis. RESULTS SAD and lesion volume demonstrated inverse linear correlations with blood flow (for SAD, y = -0.044x + 3.925, r = 0.836, P < .001; for volume, y = -0.556x + 31.574, r = 0.842, P < .001). A 10 mL/min/100 g change in flow rate produced an average 4.4 ± 0.4 mm change in SAD and an average 5.6 ± 0.5 cm(3) change in volume. Long-axis diameter was not correlated with blood flow (y = -0.7694x + 4.1899, r = 0.2173, P = .111). CONCLUSIONS The SAD and volume of radiofrequency ablation lesions have statistically significant inverse linear correlations with portal venous blood flow, with an average 4.4-mm change in SAD and an average 5.6-cm(3) change in volume for each 10 mL/min/100 g change in flow rate.