The objective of this study was to establish an ultrasound scoring system to assess the degree of fibrin tag and clot formation associated with jugular catheters in horses. A second objective was to test the hypothesis that, in healthy horses undergoing short-term anesthesia, polyurethane (PU) catheters have a different ultrasound score compared with polytetrafluoroethylene (Teflon) catheters. Twenty-three systemically healthy horses requiring anesthesia were randomly assigned PU or Teflon catheters. The mean catheter dwell time was 5 hours. Ultrasound was used to examine the jugular vein before catheter placement, before catheter removal (mean, 4 hours 45 minutes), and approximately 24 hours after catheter insertion (mean, 24 hours 42 minutes). The repeatability of the ultrasound scoring system was assessed by producing a series of randomly ordered selected still images for independent evaluation by four radiologists on two separate occasions. The interobserver and intraobserver repeatability was determined using a weighted kappa statistic, with substantial intraobserver repeatability mean ? .77 (range, .59-.91) and moderate interobserver repeatability mean ? .53 (range, .33-.67). The proportion of positive bacteriologic cultures in the polyurethane (PU) and Teflon groups was compared using Fisher's exact test, with no significant difference (P > .99) between the two groups. The distribution of the scores from the PU and Teflon groups was compared using the Mann-Whitney test, with no evidence of a difference between groups at the time of catheter removal (P = .23) and 24 hours after catheter insertion (P = .20). Further studies with a larger sample of systemically healthy horses are required to substantiate this preliminary finding.