OBJECTIVE: To characterize the structured abstracts in biomedical journals indexed in MEDLINE over a three-year period as an initial step in exploring their utility in enhancing bibliographic retrieval. DESIGN: The study examined the occurrence of structured abstracts in MEDLINE from March 1989 to December 1991, characteristics of MEDLINE records for articles with structured abstracts, editorial policies of six selected MEDLINE journals on structured abstracts, and a sample of twenty-five structured abstracts from the six journals. RESULTS: The study revealed that the number of structured abstracts in MEDLINE and the number of MEDLINE journals publishing structured abstracts increased substantially between 1989 and 1991. On average, articles with structured abstracts had more access points (Medical Subject Heading [MeSH] terms and text words) than MEDLINE articles as a whole. The average length of the structured abstract was greater than the average length of all abstracts in MEDLINE. CONCLUSIONS: The presence of structured abstracts may be associated with other article characteristics that lead to the assignment of a higher average number of MeSH headings or may itself contribute to the assignment of more headings. The variations in the structured-abstract formats prescribed by different journals may complicate the exploitation of these abstracts in bibliographic retrieval systems. More research is needed on a number of questions related to the quality and utility of structured abstracts.