Pseudomembranous colitis is mostly related to antibiotics and it presents symptoms of diarrhea, abdominal pain, fever, hypoalbuminemia and hypovolemia. Diarrhea is the most common manifestation, but in geriatric patients, symptoms of pseudomembranous colitis can be different from those of usual cases and the course of disease can be more aggressive. For these reasons, it can be misdiagnosed. So physicians must take pseudomembranous colitis into consideration in older patients with acute abdominal pain, who have been treated with antibiotics. We reported a case of an older patient with pseudomembranous colitis which was misdiagnosed as acute appendicitis with perforation.