According to the Wichita Eagle, a man who was treated at a local hospital for an injury to the back of his head was found to have a blood-alcohol concentration of 0.57. That is seven times the legal limit for driving, and is also significantly above the normal limit for staying alive, generally said to be about 0.40.
The man needed eight staples to close a gash on the back of his head. He accused his girlfriend of hitting him in the head, but the report said police had no evidence of that (other than the gash). She said she had heard a "loud thump" and found him lying on the floor, and in the apparent absence of any motive for whacking him, plus the ginormous BAC, police apparently believed it more likely he had just fallen over.
If you are wondering whether that is the highest BAC ever recorded, the answer is no.
According to The Smoking Gun, in late 2007 an Oregon woman was measured at 0.72 after police found her unconscious in her car, which she had driven into a snowbank about 50 feet ahead of a sign reading "Don't Drink and Drive." That beat the previous U.S. record, set in October of 2003 by an Indiana man who was measured at 0.69 when police found him sitting in the passenger seat of a car that itself was sitting in a ditch. He does not appear to have been driving, at least, although the driver (who was found stumbling around in the woods) was also very drunk.
Internationally, as I wrote a while back, news reports in 2005 said that a Bulgarian man had "astounded doctors" by remaining conscious and talking although they had measured his BAC at 0.914. The source for that was a CBC report quoting the Sofia News Agency, so it might or might not be reliable - although it is hard to see why the Bulgarians would make that up, unless they just wanted to be the best in the world at something for once.
Sadly, even that may fall short of the record. A 1984 article in the journal Archives of Internal Medicine reported that a patient had survived an "unprecedented" BAC of 1.50. I did not pay to get that article out of their archives, but the study is referred to in this 1999 article in the Journal of Studies on Alcohol and Drugs and the abstract (or the report itself) is available here. (It doesn't say whether or not this guy was Bulgarian.) If you are thinking of trying for the record, please consider that the man's life was only saved by "intravenously administered fructose," which doesn't sound so bad, and "peritoneal dialysis," which does.