The paper describes results of an empirical study, where some hypotheses about the impact of reuse on defect-density and stability, and about the impact of component size on defects and defect-density in the context of reuse are assessed, using historical data (data mining) on defects, modification rate, and software size of a large-scale telecom system developed by Ericsson. The analysis showed that reused components have lower defect-density than non-reused ones. Reused components have more defects with highest severity than the total distribution, but less defects after delivery, which shows that that these are given higher priority to fix. There are an increasing number of defects with component size for non-reused components, but not for reused components. Reused components were less modified (more stable) than non-reused ones between successive releases, even if reused components must incorporate evolving requirements from several application products. The study furthermore revealed inconsistencies and weaknesses in the existing defect reporting system, by analyzing data that was hardly treated systematically before.