"BioMed Central calculates unofficial impact factors for journals that have published a significant number of research articles but do not yet have an official impact factor. These unofficial impact factors use the same citation data and methodology used by Thomson Scientific's official impact factor calculations.

Newly calculated unofficial 2006 impact factors show a significantly increased citation rate for BioMed Central journals. For example, BMC Biology and BMC Medicine, the flagship journals of the BMC series, both increased their unofficial impact factors significantly. The 2006 unofficial impact factor for BMC Biology is 4.43 (up from 3.81 in 2005) while for BMC Medicine the 2006 figure is 4.17 (up from 2.52 in 2005).