In this day of "faster, cheaper, better" release cycles, software developers must focus enhancement efforts on those modules that need improvement the most. Predictions of which modules are likely to have faults during operations is an important tool to guide such improvement efforts during maintenance. Tree-based models are attractive because they readily model nonmonotonic relationships between a response variable and predictors. However, tree-based models are vulnerable to overfitting, where the model reflects the structure of the training data set too closely. Even though a model appears to be accurate on training data, if overfitted, it may be much less accurate when applied to a current data set. To account for the severe consequences of misclassifying fault-prone modules, our measure of overfitting is based on expected costs of misclassification, rather than the total number of misclassifications. In this paper, we apply a regression-tree algorithm in the S-Plus system to classification of software modules by application of our classification rule that accounts for the preferred balance between misclassification rates. We conducted a case study of a very large legacy telecommunications system, and investigated two parameters of the regression-tree algorithm. We found here that minimum deviance was strongly related to overfitting, and can be used to control it, but the effect of minimum node size on overfitting is ambiguous.