RadLex is a comprehensive lexicon—a unified language of radiology terms—for standardized indexing and retrieval of radiology information resources. With more than 68,000 terms, RadLex satisfies the needs of software developers, system vendors and radiology users by adopting the best features of existing terminology systems while producing new terms to fill critical gaps. RadLex also provides a comprehensive and technology-friendly replacement for the ACR Index for Radiological Diagnoses. It unifies and supplements other lexicons and standards, such as SNOMED-CT and DICOM. RadLex development has received support from both the National Institute of Biomedical Imaging and Bioengineering (NIBIB) and the cancer Biomedical Informatics Grid (caBIG) project.