Controlled Medical Terminologies (CMTs) are indispensable tools for present medical information systems and medical informatics researches. Concept-oriented architecture and multiple-hierarchy have been accepted as two of the desiderata of contemporary CMTs. A common problem for informaticians working with class-based methods for terminologies is to find the Most Relevant Common Ancestor (MRCA) for a group of concepts, the common ancestor which has the closest semantic distance to all the given concepts. Finding an ancestor concept to summarize a group of concepts is required to map concepts from the knowledgebase to those of appropriate granularity in CMTs. However, manually exploring the hierarchical relationships and determining the MRCA are daunting tasks due to the massive size, the multiple hierarchies and in some case the cycles of the hierarchical networks of terminologies. In our study, we developed a web-based visualization tool, the Common Ancestry Summarizer (CAS), that graphically displays the hierarchical relations of multiple concepts within CMTs, and also assist users to identify the MRCA of these concepts by identifying the Lowest Common Ancestors (LCAs). The CAS has been shown useful in organizing classes for the controlled terms used in clinical guidelines (e.g. "the bottom-up method").