OBJECTIVE The aim was to investigate the effect of increased left ventricular filling pressure on the pulmonary vein flow (PVQ) pattern. METHODS Pulmonary vein flow was recorded using an ultrasonic transit time flow meter in six anaesthetised dogs. Mean left atrial pressure was increased by stepwise volume loading from 7.8(SEM 1.3) to 18.9(1.9) mm Hg (p < 0.01). RESULTS With loading the PVQ signal developed several characteristic positive and negative waves which corresponded to directionally opposite pressure waves in the left atrium. There was a marked increase in the amplitude of the PVQ signal: peak flow increased from 165(50) to 310(38) ml.min-1 (p < 0.01), while minimum flow decreased from 49(37) to -61(23) ml.min-1 (p < 0.01). The minimum value of PVQ occurred during early ventricular systole, corresponding to the left atrial C wave. With progressive loading there was an increasing deceleration of flow during atrial contraction. To quantify the effect of atrial contraction and the C wave on the flow pattern a ratio was calculated between the integrated flow from the start of atrial contraction to the nadir of the x descent and the integrated flow during the rest of the cardiac cycle. This ratio decreased from 0.40(0.06) to 0.11(0.07) with loading (p < 0.01). In each experiment this flow ratio varied inversely with mean left atrial pressure (regression coefficients between 0.66 and 0.97). CONCLUSIONS Volume loading caused marked changes in the pulmonary vein flow pattern. The PVQ waves reflected the pressure waves in the left atrium. The relative flow during atrial contraction varied inversely with mean left atrial pressure. Further studies should be done to determine whether this index reflects left ventricular filling pressure under different conditions.