Venous cannulation, although a minor procedure, is often painful. The present study was planned to evaluate the efficacy of a diclofenac transdermal patch placed over the venepuncture site in decreasing the pain of cannulation. Seventy-two adults undergoing elective surgery were included in this randomised, prospective, double-blind, placebo-controlled study. Patients were divided into three equal groups. The Control group had a placebo adhesive patch placed on the both the dorsum of hand and the buttock; the Diclofenac-Buttock group had a placebo patch placed on the dorsum of the hand and a diclofenac transdermal patch on the buttock; the Diclofenac-Hand group had a diclofenac transdermal patch placed on the dorsum of hand and a placebo patch on the buttock. The patches were applied 1 h before cannulation. An 18G cannula was used for all venous cannulations. Pain during cannulation was assessed on a non-graduated 10-cm visual analogue scale. Median [interquartile range] pain scores were 3.0 [2.0–4.0] in the Diclofenac-Hand group, 5.0 [4.3–7.8] in the Diclofenac-Buttock group and 6.5 [4.5–7.0] in the Control group, p < 0.05. The numbers needed to treat were six and two in the Diclofenac-Buttock and Diclofenac-Hand groups, respectively. The application of a diclofenac transdermal patch at the cannulation site appears to be effective in decreasing cannulation pain.