PubMed recently attained a major milestone when the 20 millionth citation was added to the database on July 27, 2010. When access became free on June 26, 1997, usage almost immediately tripled, and today almost one billion searches are performed in PubMed annually.
Coincidentally, PubMed Central, the NIH free digital archive for life sciences and biomedical literature, added its 2 millionth full-text article on the same day. Since its debut in February 2000 it has grown to a collection of more than 650 journals which deposit their complete content, as well as some historic journal collections from the 1800s. This content is linked to the citations that appear in PubMed database searches.
Source: NLM Tech Bull. 2010 Sep-Oct;(376):e3.