This purpose of this study was to develop evidence of a relationship between law libraries and private law firm profitability for law library administrators to use when making strategic decisions that influence the value of their libraries. The highest ranked administrator at each private law firm listed on the 2008 Am Law 200 was invited to complete an online benchmarking survey. The adjusted sample population totaled 179 firms. Fifty-one valid surveys were completed for a 28.5% response rate. Descriptive and statistical analyses were conducted using 26 independent variables (law library characteristics) and a single dependent variable, Revenue per Equity Partner, developed from data published for the Am Law 200. The most significant contributions of this study are: development of important law library financial and return on investment benchmarks; a listing of characteristics that have been empirically shown to impact law firm productivity; identification of optimum reporting structure for the law library administrator. Six characteristics positively impact Revenue per Equity Partner: to whom the library Administrator reports, number of library staff per library, number of Library staff per library, range in hourly bill rate for library staff time, practice areas most often supported. Two monetary measures were also established. The cost benefit of an Am Law library to its firm is $1.00 : $1.68. Each Am Law Library staff member is worth $295,000 in Revenue per Equity Partner to a firm. Law library practitioners can use the results to support evidenced-based strategic decision making in the administration of any private law firm library. Faculty and students in law librarianship programs will have a greater understanding of how to manage law libraries and collections to provide maximum value to their law firms. Benefits to library and information science research include validation of the research design and benchmarking as a theoretical framework for conducting research into ways libraries can deliver value and return on investment to their sponsors. This research design can be generalized and replicated in future studies. It demonstrates how rank can be used to operationalize relative measures of value for research purposes. Findings from future studies in both for-profit and non-profit settings using appropriate measures of rank may form the basis for development of a theory concerning the relationship between a library and their sponsor's success.