As of late March, PubMed now goes back to 1947 and the 20 millionth citation was recently added!
Many publishers have added even older citations to PubMed. Some key public health journals are in
PubMed back to Volume 1:
* American Journal of Public Health: Volume 1, 1911
* Public Health Reports: Volume 1, 1878
* American Journal of Epidemiology: Volume 1, 1921
For more details, see: http://www.nlm.nih.gov/news/biomedical_db_moves.html