The damaging effect of enteric-coated and plain naproxen tablets on the gastric mucosa was studied in 12 healthy subjects before and after 7 days' treatment in a randomized, double-blind, double-dummy, cross-over trial. Both formulations of the drug caused mucosal lesions, but the extent of the damage was significantly decreased after enteric-coated naproxen as compared with plain tablets. The subjects' preference was significantly in favour of the enteric-coated naproxen tablets. The plasma naproxen concentration was significantly higher after treatment with enteric-coated naproxen than after treatment with plain tablets. In conclusion, the results of the study indicate that naproxen might damage the gastric mucosa by local and systemic effects and that the local effect might be prevented by enteric coating of the tablets.