The object-oriented paradigm is presently considered the one which best guarantees the investments for renewal. It allows to produce software with high degrees of reusability and maintainability, satisfying in a certain measure also quality characteristics. These features are not obviously automatically guaranteed by the simple adoption of an object-oriented programming language, a process of re-analysis is needed. In this view, several methods for reengineering old applications according to the object-oriented paradigm were defined and proposed. In this paper, a method and tool (C2O2, COBOL to Object-Oriented) for analyzing COBOL applications in order to extract its object-oriented analysis is presented. The tool identifies classes and their relationships by means of a process of understanding and refinement in which COBOL data structures are analyzed, converted in classes, aggregated, and simplified semiautomatically. The algorithm is also capable of detecting data structures which can cause problems passing to the next millennium, as demonstrated with an example.