The effect of atrial fibrillation on pulmonary venous flow patterns is still not well known. Twenty-four patients in atrial fibrillation and 21 patients in sinus rhythm were studied by transoesophageal echocardiography. In ninety-five percent (20/21) of sinus rhythm patients, the early systolic wave due to atrial relaxation or reverse wave due to atrial contraction could be distinguished on pulsed Doppler tracings by transoesophageal echocardiography. However, there was no early systolic wave and/or reverse at the end of diastole in any atrial fibrillation patients. In atrial fibrillation patients without mitral regurgitation (n = 14), the onset of systolic flow was delayed (165 +/- 38 vs 50 +/- 46 ms, P < 0.05), and systolic peak velocities, time-velocity integrals and systolic fractions were reduced (31 +/- 13 vs 54 +/- 17 cm.s-1, P < 0.05; 5 +/- 2 vs 13 +/- 6 cm, P < 0.05 and 36 +/- 8 vs 61 +/- 15%, P < 0.05, respectively) as compared to those in sinus rhythm. Significant mitral regurgitation (n = 10) reduced systolic velocity parameters considerably in atrial fibrillation patients but the diastolic flow parameters were not significantly different between sinus rhythm and atrial fibrillation patients. Stepwise multiple regression analysis identified atrial fibrillation as an important independent predictor for changes in systolic flow parameters. The R-R interval is also an important factor for diastolic flow parameters. Thus, the present study demonstrates that atrial fibrillation significantly modifies pulmonary venous flow pattern and is an important factor for systolic flow parameters. Significant mitral regurgitation can further modify systolic flow pattern in atrial fibrillation patients.