Libraries, especially those supporting the sciences, continually face the problem of selecting appropriate new books for their users. Traditional collection development techniques include the use of librarian or user subject specialists, user recommendations, and approval plans. These methods of selection, however, are most effective in large libraries and do not systemically correlate new book purchases with the actual demands of users served. This paper describes a statistical method for determining subject strengths and weaknesses in a library book collection in relation to user demand. Using interlibrary loan borrowing and book acquisition statistics gathered for one fiscal year from three health sciences libraries, the authors developed a way to graph the broad and narrow subject fields of strength and potential weakness in a book collection. This method has the advantages of simplicity, speed of implementation, and clarity. It can also be used over a period of time to verify the success or failure of a collection development program. Finally, the method has potential as a tool for use by two or more libraries seeking to improve cooperative collection development in a network or consortium.