Lambs from a Finnish Landrace flock at Moredun Research Institute are frequently affected by a diarrhoea which does not respond to anthelmintic. Three different drugs have been used (levamisole, albendazole and ivermectin) and there is no history of drug resistance at the site. By the normal rules of differential diagnosis, nematodiasis would be ruled out as a cause of the diarrhoea. However, seasonal, year-to-year and apparent breed differences in incidence led to the suggestion that outbreaks in autumn were caused by hypersensitive immune responses to the continuous ingestion of infective nematode larvae (Suttle, 1994). To maintain the nematode connection and distinguish the condition from those associated with anthelmintic resistance, it was described as an anthelmintic unresponsive diarrhoea (AUD). If the proposed aetiology is correct, one would expect hoggs to be affected in their second year of exposure to nematodes while lactating ewes with relaxed immunity might be less vulnerable. Furthermore, administration of anthelmintic in a slow release bolus might control AUD where monthly drenching failed.