OBJECTIVE: To determine the prevalence of right atrial thrombi in children with cancer and indwelling catheters. STUDY DESIGN: We systematically examined 156 children with cancer and indwelling catheters for the presence of a right atrial thrombus when they underwent routine screening of cardiac function by two-dimensional echocardiography. RESULTS: Thirteen children (8.8%) had right atrial thrombi. Of the 13 children, 6 had thrombi adherent to the right atrial wall, and in 5 of these 6 children the clots were considered large enough to require intervention: 2 children with obstruction of venous or tricuspid valve inflow underwent right atriotomy and thrombus removal; they recovered and remained well. The other 3 children had moderate-sized thrombi and were treated with oral anticoagulants; their clots stabilized (2 children) or regressed (1 child). The remaining 7 children had thrombus on the tip of the catheter: 5 of these children were observed; the thrombi spontaneously resolved in 2 of them and did not change in size in the other 3. In the sixth child, the catheter malfunctioned 2 weeks after discovery of the clot and the catheter was removed. The seventh child was treated with warfarin and the clot decreased in size. Thrombi were detected in a greater proportion of children with catheter tips in the right atrium versus the superior vena cava, and in a greater proportion of children with acute lymphoblastic leukemia versus other diagnoses. There was no association between the presence of a clot and duration of time the catheter was in place, the number of catheters placed, treatment with asparaginase, or treatment with total parenteral nutrition. CONCLUSION: The incidence of right atrial thrombi in children with indwelling catheters may be higher than was previously appreciated. (J PEDIATR 1996;128:841-6)