Since the introduction of object-oriented (OO) development techniques into industrial practices for software development, many Function Point (FP) technique adaptations have been proposed to improve estimations on the size of a software application. Most research works only deal with OO modifications to the previous version of the FP Counting Practices Manual (4.1) or they do not include some important UML specifications such as the composition relationship between classes. In this paper, we propose rules to identify Internal Logic Files (ILF) and External Interface Files (EIF) using analysis class diagrams. These rules were defined in accordance with the recommendations included in the FP Counting Practices Manual 4.2.1. We also present the results obtained by applying our rules to software size estimation case studies performed with undergraduate and graduate students. These results have proved our proposal to be at least equally accurate and consistent with the original FP technique. © 2009 Springer-Verlag.