Catheter ablation in children requires placement of multiple large bore femoral venous sheaths and catheters. Magnetic resonance (MR) angiography was used to evaluate the effect of indwelling lines on femoral venous blood flow. MR angiography has become an accepted non-invasive technique in evaluating blood vessel anatomy in adults. Between 10/93 and 3/94, eighteen patients scheduled for catheter ablation underwent MR venous angiography. Two-dimensional time of flight MR angiography was performed 24 to 72 hours after catheterization on all patients (13 patients had pre-catheterization studies). All patients received intravenous heparin during the procedure and had aspirin therapy instituted after ablation. Nineteen catheter ablations and MR angiography studies were performed on the 18 patients (1 patient underwent repeat ablation during the study period). There were 7 females and 11 males. The mean age was 14.91 ± 4.14 (range 8 to 21) years. Patients had three venous sheaths inserted in the left femoral vein (5, 6 and 7 French with external diameters measuring 1.7 mm, 2.0 mm and 2.3 mm respectively) and one sheath in the right femoral vein (7 French). Five patients (26%) had altered venous flow (3 complete obstruction and 2 partial) following catheterization. None of these patients experienced symptoms or complications. One of these five patients had long term follow-up MR angiography which revealed resolution. There is a moderate occurrence (26%) of venous obstruction following catheter ablation but no related complications. MR venous angiography provides a rapid non-invasive method to evaluate venous flow abnormalities and possibly detect patients at risk for complications.