Software maintenance is often described as the most costly activity relating to software. Understanding software modification distribution could therefore greatly benefit information system (IS) managers. Current models mostly depict this distribution as homogeneous with a decreasing rate of modifications over time. The in-depth case study reported in this paper challenges this view. A detailed analysis of software modification by type, combined with interviews, shows that in the state-of-the-art IS that was examined, there were three distinct periods during the initial 29-month period studied. In the first period, the software was stabilized within the framework of its original specifications. Software modifications during this period centered on corrective modification, similar to software testing. In the second period, the software was improved: new functions were added within the original framework. In the third period, the IS was expanded beyond its original specifications by adding many new applications.