Clinical and hematological changes observed on presentation of 47 horses referred to the Ontario Veterinary College with acute idiopathic colitis were analyzed for their prognostic features. Cases of acute enterocolitis were characterized by fever, dehydration, abnormalities of serum electrolyte concentrations, azotemia, hypoalbuminemia, and increased serum concentrations of muscle enzymes. Severely dehydrated horses were seven times more likely to die or be euthanized than those that were not dehydrated. Other factors associated with failure to survive included the following: increased hematocrit, increased number of band neutrophils, increased serum creatinine and urea concentrations, and decreased blood pH and increasingly negative base excess. The results of multivariate variable analysis (stepwise logistic regression) suggested that, among the variables tested, base excess was the best predictor of death or survival. Twenty of 47 horses died or were euthanized. Reasons for death or euthanasia included: severe disseminated intravascular coagulation, unresponsiveness of severe metabolic acidosis and hypoproteinemia to treatments, and severity of colonic lesions on exploratory laparotomy. Of the surviving horses, three developed chronic laminitis (two were destroyed) and five developed jugular vein thrombosis. Fourteen of 16 horses for which subsequent histories were available returned to normal function.Early recognition of the disease, combined with early and aggressive correction of dehydration and of acid-base imbalance, may be important determinants of survival in horses with acute idiopathic colitis.