This document describes how to map Unified Medical Language System (UMLS) codes to International Classification of Diseases 10th Revision (ICD10) codes. It involves building an ontology of the ICD10 code hierarchy and using SPARQL queries over the ontology and UMLS concepts to find the most specific ICD10 code for a given set of UMLS codes describing a patient's diagnosis. An application was created to demonstrate mapping UMLS codes from an electronic medical record to the appropriate ICD10 code for billing and other purposes.