BPA stands for Bisphenol A, a chemical compound commonly found in plastic drink and food containers. Small amounts of BPA may leach from containers and be ingested. Naturally, there is much scientific debate regarding what levels are harmful to humans. For further discussion on BPA, check out http://www.thegreenguide.com/doc/114/bpa and http://www.cdc.gov/exposurereport/pdf/factsheet_bisphenol.pdf.***********EDIT: BPA may cause cancer if you consume it. However, I have heard that the only way to get the BPA to leach out is to drink boiling water out of the bottle. But check the facts.I wonder what's worse, consuming BPA or drinking boiling water?!? The BPA leachs out at high temperatures. For this reason the old Nalgenes said to NOT put in the dishwasher.