It is known that during maintenance activities system quality tends to degrade. This work presents a field investigation carried out on two industrial projects with the aim of comparing the effectiveness of the "full reuse" maintenance process with an "iterative enhancement" one. The results of the work confirm the greater effectiveness of the first process in slowing down the degradation of software quality. Furthermore, through the analysis of both process and product metrics, we show that monitoring maintenance performances is a poor indicator of system quality degradation. In fact, when the reduction of maintenance performance becomes large, quality has degraded considerably and to a point that ordinary maintenance may no longer be considered.