Abstract Background Although several potential risk factors have been discussed, risk factors associated with bacterial colonization or even infection of catheters used for regional anaesthesia are not very well investigated. Methods In this prospective observational trial, 198 catheters at several anatomical sites where placed using a standardized technique. The site of insertion was then monitored daily for signs of infection (secretion at the insertion site, redness, swelling, or local pain). The catheters were removed when clinically indicated (no or moderate postoperative pain) or when signs of potential infection occurred. After sterile removal they were prospectively analyzed for colonization, defined as > 15 colony forming units. Results 33 (16.7%) of all catheters were colonized, and 18 (9.1%) of these with additional signs of local inflammation. Two of these patients required antibiotic treatment due to superficial infections. Stepwise logistic regression analysis was used to identify factors associated with catheter colonization. Out of 26 potential factors, three came out as statistically significant. Catheter placement in the groin (odds-ratio and 95%-confidence interval: 3.4; 1.5–7.8), and repeated changing of the catheter dressing (odds-ratio: 2.1; 1.4–3.3 per removal) increased the risk for colonization, whereas systemic antibiotics administered postoperatively decreased it (odds ratio: 0.41; 0.12–1.0). Conclusion Colonization of peripheral and epidural nerve catheter can only in part be predicted at the time of catheter insertion since two out of three relevant variables that significantly influence the risk can only be recorded postoperatively. Catheter localisation in the groin, removal of the dressing and omission of postoperative antibiotics were associated with, but were not necessarily causal for bacterial colonization. These factors might help to identify patients who are at increased risk for catheter colonization.