OBJECTIVE To compare efficacy and gastrointestinal (GI) tolerability of a new enteric coated formulation of naproxen (NAP-EC) with standard immediate release naproxen (NAP-STD). METHODS One hundred seventy-nine patients with osteoarthritis (OA) and one hundred seventy-six patients with rheumatoid arthritis (RA) at high risk for developing GI side effects to nonsteroidal antiinflammatory drug (NSAID) therapy were enrolled in a double blind, parallel, multicenter study. All patients had either discontinued as NSAID during the previous one year or required cotreatment with antiulcer drugs for control of GI complaints related to NSAID use. The treatments were evenly divided in both diagnostic cohorts. RESULTS Except for minor differences in alcohol consumption, baseline characteristics of patients in both treatment groups were statistically similar. Both naproxen formulations were highly efficacious by all variables of disease activity when changes were measured from baseline. No statistically significant between formulation difference was found in the primary efficacy variable, overall disease activity. Overall, between formulation differences in efficacy measures were few, though most favored NAP-STD. GI complaints were reduced by 15% (51% NAP-EC vs 60% NAP-STD, p = 0.077) and GI complaints thought to be drug related were reduced by 36% (16% NAP-EC vs 25% NAP-STD, p = 0.024). Withdrawals due to GI complaints were reduced by 37% in the NAP-EC group (12% NAP-EC vs 19% NAP-STD, p = 0.054), and withdrawals due to GI complaints judged to be drug related were reduced by 55% in the NAP-EC group (6% NAP-EC vs 12% NAP-STD, p = 0.025). CONCLUSIONS Enteric coated naproxen is an effective treatment for OA and RA. All observed differences in GI tolerability favor NAP-EC over NAP-STD.