The aim of this study was to identify factors that contribute significantly to the bacterial contamination of peripheral intravenous catheters in dogs and cats. Between January and June 2005, intravenous catheters were removed from 84 dogs and 15 cats at the Queen Mother Hospital for Animals, Royal Veterinary College. None of the factors under consideration was significantly associated with bacterial contamination, but 42.9 per cent of the animals with clinical signs consistent with a peripheral catheter-related infection, 34.8 per cent of the animals in which blood had been collected from the catheter immediately after its insertion, and 21.1 per cent of the animals in which a T-connector rather than a Y-connector had been used had contaminated cannulae, compared with 19.0 per cent, 19.7 per cent and 8.3 per cent, respectively, of the animals that did not have signs of such an infection, from which blood was not taken immediately, and that had a Y-connector rather than a T-connector. Binary logistic regression showed that the animals with clinical signs of a catheter-related infection were 10 times more likely to have a contaminated catheter (odds ratio [OR] 10.9, 95 per cent confidence interval [CI] 0.89 to 134) and the animals fitted with Y-connectors rather than T-connectors were 10 times less likely to have a contaminated catheter (OR 0.10, 95 per cent CI 0.008 to 1.25).