To assess the efficacy and safety of a topical formulation of 2% diclofenac in lecithin organogel in the treatment of pain associated with mild to moderate osteoarthritis (OA) of the knee. Seventy patients completed a double blind, randomized, placebo controlled, parallel group design 2 week clinical trial. Patient responses to disease-specific (WOMAC VA3.0) and quality of life (Medical Outcome Survey SF-36) health status measures were assessed. Global assessments were also made at baseline and post-treatment. The physician conducted a global assessment and range of motion of the knee at baseline and post-treatment. T tests on the aggregated WOMAC total score and aggregated subscale scores revealed significant improvement (p<0.05) on the aggregated total score and the pain, stiffness, and physical function subscales from baseline to post-treatment for the active treatment group versus the placebo group. Analysis of gain scores from the aggregated WOMAC total score and aggregated subscale scores also revealed that this improvement was significantly greater than the improvement recorded by the placebo treatment group on the aggregated total and the pain and physical function subscale scores. Other efficacy measures exhibited no significant differences between or within treatment groups. A topical formulation of 2% diclofenac in a lecithin organogel appears to have therapeutic value in patients with mild to moderate OA of the knee as determined by responses from the WOMAC (VA3.0) osteoarthritis health status measure.