Fifty patients with rheumatic mitral stenosis aged twelve to thirty-six (twenty +/- six) years were studied by two-dimensional, pulsed and color Doppler echocardiography during, seventy-two hours after, and biweekly for three months after balloon mitral valvuloplasty (BMV). Transesophageal echocardiography (TEE) done immediately after BMV (in the catheterization laboratory) detected a new atrial septal defect (ASD) in 46 (92%) patients. These measured 1 to 2 (mean 1.2 +/- 0.3) mm in diameter. Doppler color flow mapping guided the location of the ASD in most of the cases. A narrow jet of left-to-right shunt could be evaluated by pulsed Doppler studies. Velocity time integral (VTI) of the jet across one cardiac cycle and the diameter of the ASD were used to calculate the left-to-right shunt (shunt = VTI x pi (D/2)2 x heart rate). The estimated shunt was 0.04-0.39 (mean 0.20 +/- 0.10) L/minute. A repeat study at seventy-two hours revealed the defect in 40 (80%) patients. At three months, the defect persisted in only 5 (10%) cases. The mean interval of closure of ASD was 4.6 +/- 2.2 weeks. The authors conclude(1) ASD occurs commonly after BMV, (2) the septal defect and the resultant left-to-right shunt are insignificant, and (3) ASD disappears in the majority of cases by three months after BMV.