OBJECTIVE: To assess the efficacy and safety of a diclofenac hydroxyethylpyrrolidine (DHEP) patch in the treatment of symptomatic osteoarthritis (OA) of the knee joint. METHODS: A double-blind, randomised, placebo-controlled trial was carried out on 103 outpatients for 2 weeks. The main efficacy parameters were spontaneous pain and Lequesne's Index. Secondary endpoints were walking time over a standard distance, global assessment of efficacy and tolerability, and paracetamol consumption. RESULTS: The active treatment group showed a significant improvement in pain, Lequesne's Index, and the physician's and patient's global assessment of efficacy. For these parameters the difference between groups was statistically significant in favour of the DHEP patch. Adverse reactions were seen in a small number of probands and were similar in both groups. CONCLUSIONS: The results of this trial suggest that the DHEP patch appears to be an effective and safe treatment for patients suffering from symptomatic knee OA.