[The problem "diagnosis" demonstrated on the example of farmer's lung].

Using the farmer's lung as an example, the various problems in establishing a diagnosis are discussed. The "instruments" of diagnosis such as history (exposition), functional parameters, macro- and micromorphology, and laboratory findings are described. The different criteria and their value, the compatibility of informations, and the quality of definitions ("fuzzy sets") are analytically evaluated--all this finally leading to the "quality" of diagnosis and of subsequent decisions.