Antibiotics-associated diarrhea and pseudomembranous colitis are increasingly reported from some hospitals in Hanoi city, but no laboratory tests for causative agents are available. Thus, this is the first study on the proportion of Clostridium difficile infections in Hanoi, Viet Nam. The authors reported two cases with severe hospital-acquired diarrhea and community-acquired pseudomembranous colitis due to anaerobic bacteria Clostridium difficile using toxin detection test and stool culture of toxigenic Clostridium difficile. The article also provided the first bacteriological evidences on the circulation of 2 bacterial clones, Clostridium difficile A + B+ and Clostridium difficile A- B+ in antibioticsassociated diarrhea in Viet Nam.