Documents can be assigned keywords by frequency analysis of the terms found in the document text, which arguably is the primary source of knowledge about the document itself. By including a hierarchi- cally organised domain specific thesaurus as a second knowledge source the quality of such keywords was improved considerably, as measured by match to previously manually assigned keywords. In the presented ex- periment, the combination of the evidence from frequency analysis and the hierarchically organised thesaurus was done using inductive logic programming.