The present study identifies the classification systems used in 941 libraries. It explores the reasons behind the choice of the National Library of Medicine Classification by 589 libraries. Reclassification procedures were investigated through a questionnaire sent to twenty-five libraries that have changed to NLM since 1959. Statistics and replies are given on: the classification systems employed prior to reclassification; the use of broad or specific Library of Congress class numbers in the LC schedules outside the scope of the NLM schedules; the number of catalogers in each library doing the reclassification; the use of cut-off dates for retrospective materials; the adoption of MeSH headings; user preference; and cost differences. Chief reasons for the change to NLM proved to be local circumstances, currency, arrangement of subclasses by NLM, its dovetailing with the LC Classification, and reliance upon nationally centralized cataloging services.