Mapping medical concepts from a terminology system, to the concepts in the narrative text of a medical document is necessary to provide semantically accurate information for further processing steps. The MetaMap Transfer (MMTx) program is a semantic annotation system that generates a rough mapping of UMLS concepts to free medical text, but this mapping still contains erroneous and ambiguous bits of information. Since manually correcting the mapping and annotating the text chunks of a document accordingly would be an extremely cumbersome and time-consuming task, and since there has been no satisfactory tool to support it, we have developed the MapFace editor. The editor not only greatly facilitates the handling of the MMTx program, but it also provides a convenient way of navigating the annotated information gained from the MMTx output, and enables users to correct this information on both a conceptual and a syntactical level. Thus, we are able to provide reliable and well-defined information for further processing steps. Finally, a usability study was conducted to derive an informative basis for improvement.