The role of multiplane (M) transesophageal echocardiography (TEE) in the diagnosis of isolated patent ductus arteriosus (PDA) in adults and its effectiveness in the assessment of the pulmonary to systemic flow ratio were evaluated and compared with those obtained from cardiac catheterization examination. Eleven consecutive patients, ranging from 17 to 56 years old (mean of 29.5 +/- 12.0), with clinically suspected PDA were subjects of this study. A complete transthoracic echocardiographic study was performed in each patient before MTEE. MTEE with Doppler color flow mapping showed clear visualization of a ductal structure between the descending aorta and pulmonary artery with a continuous turbulent mosaic flow suggestive of PDA in all 11 patients. The pulmonary/systemic flow and vascular resistance ratios obtained by echocardiography and cardiac catheterization correlated well (r = 0.8732, P =.0004; r = 0.623, P =.04, respectively). This study demonstrated that MTEE combined with transthoracic echocardiography examination is an accurate noninvasive means in the diagnosis of PDA and assessment of the pulmonary to systemic flow and vascular resistance ratios in adult patients.