Antibiotic associated diarrhea is a common adverse effect, occurring about 5-30% of patients either early during treatment or up to two months after the cessation of the treatment. The frequency differs according to the definition of diarrhea, the inciting antimicrobial agent and the host factors such geriatric age, immunosuppression, prolonged hospitalization. Almost all agents, especially the the ones acting on anaerobes may cause diarrhea. Clinical presentations range from mild diarrhea to fulminant pseudomembranous enterocolitis. In Turkey still data about antibiotic associated diarrhea is not enough. Incidence and the factors that effect severity are not known.