The growth of indexing services has emphasized the need for more knowledge of the indexing process itself. Consistency is necessary for continuing progress in the field. This study postulates that: (1) definitions of indexer consistency should consist of the indexer's perception of indexable concepts and his choice of terminology; (2) both parts of the definition can be measured separately; (3) there will be a large difference in the degree of each; and (4) indexer consistency scores should contain both elements. For the study, five indexers read 550 journal articles and labeled the concepts discussed in each article. Findings from this exercise indicate a need for a re-examination of the problem of indexer consistency and its relation to: (1) tests of the effectiveness and efficiency of indexing languages and systems; (2) index tools and methodology; (3) index research, much of which has concentrated on terminological relationships to the neglect of concept-related problems; and (4) indexer consistency as a factor in indexer-user consistency in choice of concepts or terms for the retrieval of indexed information. (Author/SJ)