The Medical Literature Analysis and Retrieval System (MEDLARS), in operation at the National Library of Medicine (NLM), is a multipurpose system, a prime purpose being the production of Index Medicus and other recurring bibliographies.1 This two-year study concentrated on the evaluation of the demand search function of MEDLARS (ie, the conduct of retrospective literature searches in response to specific demands).2
The base of the retrospective search module consists of approximately 800,000 citations to biomedical journal articles input to the January 1964 and subsequent issues of the monthly Index Medicus. These articles have been indexed by means of a controlled vocabulary of Medical Subject Headings (MeSH). Over 3000 demand searches are now formulated annually at the NLM, additional searches being handled at regional MEDLARS centers in the United States, in the United Kingdom, and in Sweden.
Approximately 2,300 scientific journals are indexed regularly, one third exhaustively ("depth journals") at