This paper describes an open-source Java/XSLT application that allows users to analyze and manipulate XML DTDs. The application can be used to generate reference documentation, create reports comparing two DTDs, convert DTDs into Schematron, and to automatically scaffold conversion scripts.The DtdAnalyzer tool has been used at the National Center for Biotechnology Information (NCBI), a part of the National Library of Medicine, for a number of years, to help maintain the suite of NLM DTDs, and to develop new conversion applications. NCBI has recently decided to release it as a self-contained application and to move its development onto GitHub (https://github.com/NCBITools/DtdAnalyzer).The heart of the tool is a Java application which converts a DTD into an XML representation, including all the element, attribute, and entity declarations contained inside the DTD. Additionally, DTDs can be annotated with specially-formatted comments which are recognized by this converter, and these annotations are delivered in the XML output.The resulting XML can be transformed to create many useful outputs, and a basic set of those transformation stylesheets, as described above, are included with this tool.