A Review of: Trueswell, R. L. (1969). Some behavioral patterns of library users: The 80/20 rule. Wilson Library Bulletin, 43(5), 458-461. Objective – To demonstrate the relationship between library circulation and the percent of a library's holdings satisfying circulation. Design – Retrospective cohort study of library circulation data. Setting – Cambridge and Northampton, Massachusetts, United States of America. Subjects – The users of the monographic holdings of the Air Force Cambridge Research Laboratory Library and the Forbes Public Library. Methods – Trueswell compiled circulation data from the Air Force Cambridge Research Laboratory Library over a period of 5 years, ostensibly from 1964-1969. Additionally, he gathered circulation data from Forbes Public Library. Trueswell compared each respective collection of data against the entire holdings of each library, after which he converted each measure to a percentage of the whole. Main Result – Based on the collected data, Trueswell found that the percentages of both libraries’ holdings that satisfy circulation follow a power law distribution. He compared this with a previous study measuring journal circulation at a Health Sciences Library that exhibited the same pattern. He stated that these similar distributions demonstrated the "80/20 Rule." The distribution is such that any given percent of circulation will provide the percent of a library’s circulating holdings necessary to satisfy it. Additionally, Trueswell found that 75 percent of current circulation had circulated at least once within the preceding year. Conclusion – The findings have implications for core collection development, purchasing multiple copies of a given title, determining the optimal size of a library's collection, and weeding. Trueswell also submits the idea of developing regional interlibrary loan centers for books that do not circulate often, as a cost saving measure for most libraries.