In this paper we suggest and evaluate a method for predicting fault densities in modified classes early in the development process, i.e., before the modifications are implemented. We start by establishing methods that according to literature are considered the best for predicting fault densities of modified classes. We find that these methods can not be used until the system is implemented. We suggest our own methods, which are based on the same concept as the methods suggested in the literature, with the difference that our methods are applicable before the coding has started. We evaluate our methods using three large telecommunication systems produced by Ericsson. We find that our methods provide predictions that are of similar quality to the predictions based on metrics available after the code is implemented. Our predictions are, however, available much earlier in the development process. Therefore, they enable better planning of efficient fault prevention and fault detection activities.