This paper describes a natural language-based CASE tool called CM-Builder which aims at supporting the analysis stage of software development in an object-oriented framework. CM-Builder uses robust natural language processing techniques to analyse software requirements texts written in English and build an integrated discourse model of the processed text, represented in a semantic network. This semantic network is then used to automatically construct an initial UML class model representing the object classes mentioned in the text and the relationships among them. The initial model can be directly input to a graphical CASE tool for further refinement by a human analyst. CM-Builder has been quantitatively evaluated in blind trials against a collection of unseen software requirements texts and we present the results of this evaluation, together with the evaluation methodology. The results are very encouraging and demonstrate that tools such as CM-Builder have the potential to play an important role in the software development process.