The high computational complexity of existing methods for model-based diagnosis imposes a limit on the application of those methods in practical situations. Therefore, a lot of research time is spent on methods that improve the tractability of model-based diagnosis. In this paper, a new method for improving the tractability of model-based diagnosis is presented. The reasoning mechanism used is restricted to a subset of the components in the model called the focus. This focus is selected during diagnosis, based on (1) the structure of the model and the observed values at the system outputs, and (2) measurable connections in the model. Experiments have shown that focusing improves the practical applicability of model-based diagnosis. Furthermore, we will discuss a method for ordering the resulting set of diagnoses in a way analogous to determining a focus, i.e. based on the structure of the model and observations available.