Here we report a case of nonsteroidal anti-inflammatory drug (NSAID)-associated colitis with a histology of collagenous colitis in a 77-year-old woman. The patient had taken aspirin since 1993 after being diagnosed at another hospital, as having multiple cerebral infarctions. She began to suffer from intermittent diarrhea in April 1999. Serological examination showed hypoproteinemia, which indicated that she had protein-losing enteropathy. By July 1999, she had undergone colonoscopic examination four times. Biopsy specimens taken during the fourth colonoscopy revealed collagenous colitis. As the patient had been taking aspirin for 6 years, she was diagnosed as having NSAID-associated colitis with a histology of collagenous colitis. When she stopped taking aspirin, the diarrhea ceased. Three months later, the patient underwent a fifth colonoscopy. A histological examination of the biopsy specimen revealed that the collagen band had vanished. NSAID-associated colitis sometimes shows collagenous colitis histologically and is cured by withdrawing the drug. It is important to differentiate NSAID-associated colitis, even if it shows a histology of collagenous colitis, from collagenous colitis as the two diseases differ in etiology and therapy.