I would say 104-105 like others have said. Young babies however should not be runnning a fever that high, they should still have enough immunity built up from the mother. You just want to make sure the fever does not go up quickly, my daughter had febrile seizures when she was little and I learned it is not because of how high the fever is but because of how quickly the temp goes up. If it starts to rise quickly you want to gove them a tepid bath to cool the body then seek medical attention. Febrile seizures are very scary for the parents but do not cause damage to the child.
A fever is actually a good thing, our bodies raise in temperature to try to kill off any microorganisms that are causing the illness, or to prevent their further growth. You usually want to wait until a fever is 101 to give any motrin or tylenol so that the body can do it's job.
at 3:05 AM on Dec. 22, 2008