Data from the National Library of Medicine (NLM) automated Master Serials System and its MEDLINE database were used to chart the growth of NLM's serials collection and of the journals indexed in Index Medicus from 1966 to 1985. The number of live serial titles in the subset of NLM's collection examined increased 30% in the twenty years. The average number of articles per Index Medicus journal increased 56%. The average number of articles in U.S. Index Medicus journals grew more rapidly than the average number in journals published elsewhere. The NLM data provide clear evidence that the years from 1966 to 1985 saw a substantial increase in the percentage of the biomedical serial literature published in English. The period from 1966 to 1985 saw substantial but uneven growth in the number of serial titles in the NLM collection and in the average number of articles in Index Medicus journals. Although data on the number of articles published in Index Medicus journals is unlikely to reflect the number of articles in other journals, the pattern of growth in the number of serials held by NLM probably reflects trends in the universe of all biomedical serials.