Background— Mobile thrombi, not routinely recognized on transthoracic echocardiography, are frequently identified on cardiovascular implantable electronic device leads with intracardiac echocardiography (ICE) during ablation procedures. Their incidence, characteristics, and consequences have not yet been defined. Methods and Results— We used ICE to examine leads for thrombi and to measure the pulmonary artery systolic pressure in patients with a cardiovascular implantable electronic device presenting for ablation. Patient clinical characteristics, device type, and lead characteristics were correlated with presence of thrombi. Most patients had congestive heart failure (84%), with an average left ventricular ejection fraction of 40%. Thrombi were seen with ICE in 26 of 86 patients (30%) but were seen on transthoracic echocardiography in only 1 of the 26 patients. Thrombi on ICE were mobile, averaged 18±5.9 mm long by 4.4±2.3 mm wide, and were more commonly identified in the right atrium (n=25) than in the right ventricle (n=5). Thrombi were associated with higher pulmonary artery systolic pressure: 39±9 mm Hg with thrombi versus 33±7 mm Hg without thrombi (odds ratio, 1.11; 95% confidence interval, 1.03 to 1.20; P =0.01). No other characteristic assessed was associated with a significant difference in the presence of lead thrombi. Conclusions— Mobile thrombi on cardiovascular implantable electronic device leads are present in 30% of patients undergoing ablation and are readily identified with ICE despite being underrecognized with transthoracic echocardiography. Further study is warranted to determine whether lead thrombi are a clinically relevant source of pulmonary emboli in some patients with cardiovascular implantable electronic devices.