A number of sophisticated medical ontologies have been created over the past years. With their development the need for supporting the alignment of different ontologies is gaining importance. We proposed C-OWL, an extension of the Web Ontology Language OWL that supports alignment mappings between different, possibly incompatible ontologies on a semantic level. In this paper we report experiences from using C-OWL for the alignment of medical ontologies. We briefly review key concepts of the C-OWL semantics, explain the setting of the case study including some examples from the alignment and discuss the possibility of reasoning about the mapping based on the C-OWL semantics We conclude by arguing that C-OWL provides an adequate framework for aligning complex ontologies in the medical domain.