Summary A prospective, randomised, controlled clinical study was performed to compare the incidence and severity of postoperative peripheral venous thrombophlebitis associated with a single intravenous cannula used for both intra‐operative and postoperative purposes, and two cannulae, one used intra‐operatively and the other postoperatively. Sixty American Society of Anaesthesiologists (ASA) physical status I or II patients aged 18–65 years undergoing elective surgery were studied. The technique of cannula insertion was standardised. After surgery, the cannulation sites were examined daily by a blinded investigator for the presence and severity of thrombophlebitis using the Baxter Scale. The two groups were similar in terms of age, gender, weight, type and duration of surgical procedures, and drugs and fluids administered both intra‐operatively and postoperatively. The proportion of patients that developed phlebitis was significantly less in the two cannulae group (26.1%) than in the single cannula group (63.3%) (p < 0.0001). The severity of phlebitis was greater in the single cannula group than in the two cannulae group. These results indicate that the use of a dedicated cannula for postoperative use decreases the incidence and severity of postoperative, peripheral, cannula‐related phlebitis.