ABSTRACT: This phase III multicenter randomized double-blind placebo-controlled comparative study evaluated the efficacy and safety of diclofenac sodium patches for the treatment of cancer pain. The study consisted of a 2-week to 4-week open-label dose-titration phase and a 4-week double-blind phase. In the double-blind phase, patients who were expected to continue treatment of cancer pain with nonopioid analgesics alone were randomized to the diclofenac sodium patch or placebo group. Once-daily diclofenac sodium patches were started at 150 mg/day (2 patches) and could be increased up to 225 mg/day (3 patches). The primary efficacy endpoint was the time to insufficient analgesic response. Statistical analysis of the double-blind phase included data from 120 patients of the diclofenac sodium patch group and 118 patients of the placebo group. Time to insufficient analgesic response was significantly longer with diclofenac sodium patches than with placebo (P = 0.0016). The hazard ratio for insufficient response for diclofenac sodium patch vs placebo was 0.459 (95% confidence interval, 0.275-0.768). Regarding sleep quality during the double-blind phase, the proportion of patients with "very good sleep" or "good sleep" in the diclofenac sodium patch and placebo groups was 90.8% and 88.1% at the start of the double-blind phase and 81.4% and 78.6% at the final assessment, respectively. The incidence of adverse events was 60.8% (73/120) in the diclofenac sodium patch group and 60.2% (71/118) in the placebo group. Once-daily diclofenac sodium patches are effective in treating cancer pain and are well tolerated.