A method and system for generating a taxonomy for documents from tag data are provided. The method includes obtaining tag data in the form of tags on documents with tag weightings for a document and clustering the tags using the tag weightings for documents, wherein each cluster is an identified subject. The documents are associated with each identified subject and the subjects are compared to identifying relationships between subjects to build a taxonomy of subjects. A tag weighting for a document is the number of times the tag is applied to the document with a user rating of the relevance of the tag to the document. The steps are carried out automatically without user intervention.