Data on 2,028 patients with fever of unclear origin (FUO) were analyzed and evaluated with respect to diagnostic errors. The following results were obtained: Fever of unclear origin is to be expected in about one per cent of in-patients. Most cases of FUO are caused microbially (37.0%). Neoplasms (19.2%) are in the second place. Rheumatic diseases and collagenoses cause fever of unclear origin in 11.2% of the cases. In 16.8% of the cases the cause of FUO could not be determined even after extensive diagnostics. Diagnostics can be considerably shortened and become more effective, if a check list is used, which includes the 15 most frequent causes of FUO.