Software systems are affected by degradation as an effect of continuous change. Since late interventions are too much onerous, software degradation should be detected early in the software lifetime. Software degradation is currently detected by using many different complexity metrics, but their use to monitor maintenance activities is costly. These metrics are difficult to interpret, because each emphasizes a particular aspect of degradation and the aspects shown by different metrics are not orthogonal. The purpose of our research is to measure the entropy of a software system to assess its degradation. In this paper, we partially validate the entropy class of metrics by a case study, replicated on successive releases of a set of software systems. The validity is shown through direct measures of software quality such as the number of detected defects, the maintenance effort and the number of slipped defects.