Annotations are useful to semantically enrich documents and other datasets with concepts of standardized vocabularies and ontologies. In the medical domain, many documents are not annotated at all and manual annotation is a difficult process making automatic annotation methods highly desirable to support human annotators. We propose a linguistic-based and a reuse-based approach annotating medical documents by concepts from an ontology. The reuse-based approach utilizes previous annotations to annotate similar medical documents. The approach clusters items in documents such as medical forms according to previous ontology-based annotations and uses these clusters to determine candidate annotations for new items.