OBJECTIVES: Nonsteroidal anti-inflammatory drugs have dose-related adverse effects. Topical nonsteroidal anti-inflammatory drugs may offer local efficacy with low systemic drug levels. This study assessed the efficacy and safety of topical diclofenac sodium 1% gel (DSG) in mild to moderate symptomatic knee osteoarthritis. METHODS: In a randomized, double-blind, vehicle-controlled trial, 492 adults aged >or=35 years with symptomatic knee osteoarthritis of >or=6 months' duration were randomized to DSG 4 g (n = 254) or vehicle (n = 238) 4 times daily for 12 weeks. Primary efficacy outcomes at week 12 were the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) pain subscale, WOMAC physical function subscale, and global rating of disease. Secondary outcomes included these outcomes assessed after 1, 4, and 8 weeks, and pain on movement assessed using a 100-mm visual analog scale. All adverse events were recorded. RESULTS: At week 12, the DSG group had significant decreases versus the vehicle group in mean WOMAC pain (P = 0.01), mean WOMAC physical function (P = 0.001), and mean global rating of disease (P < 0.001). Efficacy outcomes significantly favored DSG versus vehicle beginning at week 1. Application site reactions occurred in 5.1% and 2.5% of patients in the DSG and vehicle groups, respectively. The incidence of gastrointestinal disorders was 5.9% with DSG and 5.0% with vehicle. CONCLUSIONS: Over a 3-month treatment period, topical treatment with DSG achieved statistically and clinically significant improvements of pain and measures of physical function in patients with knee osteoarthritis.