Journal article overlap is defined as the same journal article being indexed by two or more services. Using journal references from seventy dissertations written in the preclinical sciences, the extent of journal article overlap among Index Medicus, Science Citation Index, Biological Abstracts, and Chemical Abstracts was examined. Of the 7,969 journal references cited, 92% were indexed by at least two of these services; 591 articles were covered by only one of the services, and 55 articles were not indexed. A discussion of the advantages and costs of journal article overlap is presented.