The Andrew W. Mellon Foundation's Graduate Education Initiative (GEI) provided funding to 54 departments in the humanities and related social sciences during the 1990s to improve their PhD programs. This article estimates the aspects of PhD programs the GEI influenced and how these aspects influenced attrition and graduation probabilities. It uses survey data on entrants to PhD programs at 44 of the "treatment" departments and 41 "control" departments during a 15-year period that spanned the start of the GEI. Factor analysis is used to group more than 100 program characteristics into a smaller number of factors, and the impact of the GEI on each and the impact of each on attrition and graduation probabilities are estimated. The article estimates the routes via which the GEI influenced attrition and graduation rates and indicates which aspects of PhD programs departments should concentrate on to improve their programs' performance. (Contains 5 tables and 17 notes.)