Many antibiotics, particularly the lincomycins, may cause diarrhoea with or without enterocolitis. The pathogenesis of antibiotic-associated diarrhoea without colitis is uncertain; colloidosmotic water binding in the colon by endogenous glycoproteins undegraded by colonic bacteria is considered. Antibiotic-associated enterocolitis is now known to be due to toxin-producing clostridia, proven for Cl. difficile. Improved methods for the detection of toxin and clostridia are presently being studied. Endoscopically, pseudomembranes are characteristic but not antibiotic-specific, they may be absent or missed diagnostically. A possible role of asymptomatic clostridia-carriers in enterocolitis clustering remains to be determined. The potentially lethal course of the disease requires rapid diagnosis and therapy, with discontinuation of the antibiotic, intensive supportive measures and, at least in severe disease, oral vancomycin.