An observational study assessed the frequency of thrombi related to central venous catheters. Any central venous catheter in place for longer than 14 days was studied with ultrasound surveillance twice a week. Around 1300 catheters were studied, and 10% of them were found to have a thrombus related to the venous catheter. These catheters with thrombi were more likely to be removed for non-elective reasons than those without thrombi. To detect one thrombus 14 ultrasounds had to be performed.