Although quality is not easy to evaluate since it is a complex concept compound by different aspects, several properties that make a good object-oriented design have been recognized and widely accepted by the software engineering community. We agree that both the traditional and the new object-oriented properties should be analyzed in assessing the quality of object-oriented design. However, we believe that it is necessary to pay special attention to the polymorphism concept and metric, since they should be considered one of the key concerns in determining the quality of an object-oriented system.In this paper, we have given a rigorous definition of polymorphism. On top of this formalization, we propose a metric that provides an objective and precise mechanism to detect and quantify dynamic polymorphism. The metric takes information coming from the first stages of the development process giving developers the opportunity to early evaluate and improve the quality of the software product. Finally, a first approach to the theoretical validation of the metric is presented.