The spore-forming anaerobe Clostridium difficile has become a serious enteropathogen. Changes in the composition of natural intestinal flora, mainly due to antibiotic therapy, permit its colonization of, and multiplication in, the colon. The disease is caused by (entero)toxin A and (cyto)toxin B, and infection ranges from asymptomatic carrier state and mild diarrhea to pseudomembranous colitis. The clinical diagnosis is made by observing inflammatory, sometimes bloody, diarrhea and by the colonoscopic detection of epithelial necrosis, ulceration, and, in the advanced state, pseudomembrane formation. The laboratory supports the diagnosis by detecting toxin A and/or B by an enzyme-linked immunoassay with high specificity, but sometimes less sensitivity than with the cytotoxin assay in tissue culture cells. Fecal leukocytes or fecal lactoferrin may be found. Culture for the isolation and identification of toxigenic C. difficile is time consuming but necessary for epidemiological studies. Polymerase chain reaction (PCR) tests have been tested for detection of the toxin B gene directly in stool. Therapy consists of stopping all systemic antibiotic treatment and the use of oral metronidazole or vancomycin. There may be more relapses after vancomycin therapy, and the increasing vancomycin resistance of Enterococcus is worrisome. Prevention, especially of nosocomial spread, requires isolation and enforced handwashing. For epidemiological studies, the bacteria can be typed by molecular DNA analyses, including PCR, protein electrophoresis, and immunological tests.