Use case modeling is a much used technique for eliciting and documenting functional requirements. The quality of the use cases may have an important impact on the development project and on the resulting software product. This paper presents an empirical study of the changes that were made to the use case models of a large software project during the analysis phase. The results show which are the most difficult aspects of use case modeling in this project context, and show how the quality of the functional requirements, in particular correctness, completeness and clarity, was improved through the use case modeling process.