Clinical and laboratory findings are recorded from a series of 15 cases (aged one to 16 years) of sudden-onset chronic diarrhoea with weight loss, progressing in many cases to emaciation and death, associated with the emergence of fourth stage cyathostome (trichoneme) larvae in large numbers through the colonic and caecal mucosae. Apart from a single incident in mid-October, disease only occurred in winter and spring (January to May). Characteristic features included hypoalbuminaemia, increased α and β plasma globulin levels and neutrophilia without left shift. Faecal egg counts were frequently negative and eosinophilia and anaemia were not features. Horses treated in the early stages responded well to larvicidal doses of fenbendazole (60 mg/kg bodyweight) but in cases where this therapy was delayed for weeks or months the prognosis was poor.