Mycotoxicoses are intoxications caused by ingestions of foodstuffs contaminated with mycotoxins, i.e. toxic secondary metabolites of microscopic filamentous fungi (moulds). By field observations and by experimental testing, toxins or toxic strains of more than 100 species of fungi have been encountered. However, causal associations have so far only been established for a small number of mycotoxicoses in farm animals, and the more important mycotoxicoses are aflatoxicosis, facial eczema, mycotoxic nephropathy, and estrogenic syndrome. A full assessment of the impact of mycotoxins on the health of farm animals can hardly be made at present, because the amount of surveillance data from mycotoxicoses is very limited, due mainly to inadequate diagnostic criteria. A new set of criteria for the diagnosis of mycotoxicosis is proposed, which has been successfully applied in the causative study of mycotoxic porcine nephropathy.