Background/Aims : The direct toxic effects of antibiotics on the intestine can alter digestive functions and cause pathogenic bacterial overgrowth leading to antibiotic-associated diarrhea (AAD). Clostridium difficile (C. difficile) is widely known to be responsible for 10-20% of AAD cases. However, Klebsiella oxytoca, Clostridium perfringens, Staphylococcus aureus, and Candida species might also contribute to AAD. Methods : We prospectively analyzed the organisms in stool and colon tissue cultures with a C. difficile toxin A assay in patients with AAD between May and December 2005. In addition, we performed the C. difficile toxin A assays using an enzyme-linked fluorescent assay technique. Patients were enrolled who had diarrhea with more than three stools per day for at least 2 days after the initiation of antibiotic treatment for up to 6-8 weeks after antibiotic discontinuation. Results : Among 38 patients (mean age 59±18 years, M:F=18:20), the organism isolation rates were 28.9% (11/38) for stool culture, 18.4% (7/38) for colon tissue cultures and 13.2% (5/38) for the C. difficile toxin A assay. The overall rate of identification of organisms was 50.0% (19/38). Of the five patients that had a positive result by the C. difficile toxin A assay, two had no organism isolated by the stool or colon tissue culture. The organisms isolated from the stool cultures were C. difficile (4), Klebsiella pneumoniae (K. pneumoniae) (3), Candida species (3), and Staphylococcus aureus (1). C. difficile (4) and K. pneumoniae (3) were isolated from the colon tissue culture. Conclusions : For C. difficile negative AAD patients, K. pneumoniae, Candida species, and Staphylococcus aureus were found to be potential causative organisms.