Transesophageal Doppler color flow imaging was performed in 19 adult patients (mean age 35 years) with an atrial septal defect demonstrated by cardiac catheterization or at surgery, or both. The transesophageal study correctly identified and classified 19 of 19 shunts in contrast to 16 of 18 shunts identified by the transthoracic approach. The area of the atrial septal defect was calculated by assuming it to be circular and taking the maximal Doppler color flow jet width at the defect site as its diameter. The pulsed Doppler sample volume was placed parallel to the shunt flow direction at the defect site to obtain the mean velocity and flow duration. From these values, the shunt volume was calculated as a product of the defect area, mean velocity, flow duration and heart rate. The calculated shunt flow volume obtained by transesophageal study showed a good correlation with shunt flow volume (r = 0.91, p < 0.001) and pulmonary to systemic blood flow ratio (r = 0.84, p < 0.001) obtained at cardiac catheterization. The size of the defect by transesophageal Doppler color flow mapping correlated fairly well with the size estimated at surgery (r = 0.73, p = 0.004). It is concluded that transesophageal Doppler color flow imaging is useful in the detection and classification of atrial septal defects and in the assessment of shunt volumes. © 1990, American College of Cardiology Foundation. All rights reserved.