Use Cases are an effective technique to express Functional Requirements of a system in a very simple and easy-to-learn way. Use Cases are mainly composed of Natural Language (NL) sentences and the use of NL to describe the behavior of a system is always a critical point, due to the inherent ambiguities originating from the different possible interpretations of NL sentences. We discuss in this paper the application of analysis techniques based on a linguistic approach to detect, within requirements documents, defects related to such inherent ambiguity. Starting from the proposed analysis techniques we will define some metrics that will be used to perform a quality evaluation of requirements documents. Some available automatic tools supporting the linguistic analysis of NL requirements have been used to evaluate an industrial Use Cases document according to the defined metrics. A discussion on the application of linguistic analysis techniques to support semantic analysis of Use Cases is also reported. 1.