A retrospective analysis of the clinical and laboratory findings from 51 adult horses with chronic diarrhoea revealed that the most common conditions were larval cyathostomiasis (14 cases), idiopathic chronic colitis (nine cases) and alimentary lymphoma (five cases). Five animals had diarrhoea as a result of non-alimentary disease. A diagnosis was reached in 37 cases, but only 15 were made ante mortem. Among the 18 animals (35 per cent) which survived, there were five cases of larval cyathostomiasis, one case of colonic impaction and 12 cases were undiagnosed. The most frequent abnormalities detected in blood samples from the horses were neutrophilia, hypoalbuminaemia, hyperglobulinaemia and high alkaline phosphatase activity. Evidence of carbohydrate malabsorption was found in 16 of 28 cases in which oral glucose tolerance tests were performed. No diagnostic specificity was apparent in either the clinical signs or the laboratory findings.