Sixteen cases of acute idiopathic toxaemic colitis developed in a veterinary hospital over a period of three years. Before the onset of colitis, 15 horses had received antibiotics, 11 had undergone general anaesthesia and various surgical procedures, and 10 had been treated with non-steroidal anti-inflammatory drugs. The horses had acute onset, profuse watery diarrhoea, profound depression, mild to moderate abdominal pain, reduced intestinal borborygmi, tachycardia, dehydration and endotoxic shock. Leucopenia, neutropenia and pyrexia were common early indicators of impending colitis. Metronidazole appeared to be an effective treatment; eight horses treated with metronidazole survived whereas five of seven horses that received other treatments, but no metronidazole, died or had to be euthanased. The aetiology of the colitis could not be determined, but the clinicopathological features resembled those of colitis attributed to an intestinal overgrowth of Clostridium perfringens type A. No Salmonella species were isolated from 52 samples of faeces, colonic contents and colonic mucosa which were collected from the horses antemortem and postmortem.