One hundred and twenty-three patients with osteoarthritis (n = 50) or rheumatoid arthritis (n = 73) were enrolled in a 6-week, double-blind, randomized, controlled, parallel trial comparing enteric-coated naproxen with standard naproxen. Ninety-eight patients subsequently entered a 20-week, open-label trial of enteric-coated naproxen. The study demonstrated that naproxen in both its standard formulation and its new enteric-coated formulation is a highly effective form of therapy for osteoarthritis and rheumatoid arthritis. The tolerability profiles of the two formulations were similar in terms of the types of complaints reported. It is concluded that enteric-coated naproxen is an efficacious and well-tolerated formulation for the treatment of osteoarthritis and rheumatoid arthritis.