Two cases of colitis associated with mefenamic acid were reported. Case 1 was a 69 -year-old woman who had taken mefenamic acid 500 mg three times daily for trigeminal pain. One year later she developed diarrhea, bloody stool, and edema. Laboratory findings revealed severe anemia and hypoproteinemia. Increased protein loss from the intestine was confirmed by ¹²⁵I-PVP test. Barium enema showed thumb-printing in the descending colon and colonoscopy showed reddish-edematous mucosa with hemorrhage in the colon and rectum. Biopsy specimen revealed inflammatory infiltrates and epithelial erosions. These abnormalities disappeared immediately after withdrawal of mefenamic acid, and lymphocyte stimulation test induced by mefenamic acid was positive. Case 2 was a 68-year-old woman who had taken mefenamic acid 500 mg three times daily for knee joint pain intermittently. Ten years later she developed diarrhea and a weight loss of 6 kg for 3 months. Laboratory findings revealed mild anemia and hypo-proteinemia. Barium enema showed no abnormality, but spotty reddness with hemorrhage was seen throughout the colon endoscopically. Histological finding of biopsy specimen was almost similar to that in case 1. Repeated endoscopy was normal immediately after withdrawal of mefenamic acid. Challenge test revealed the recurrence of the similar abnormalities. Clinical features and etiology of mefenamic acid induced colitis were discussed in our two cases and other 10 cases previously reported in the literature. More attention should be paid to the possibility that usual dose of mefenamic acid for long term can cause colitis.