According to a scholarly article in the Journal of the American Academy of Psychiatry and the Law, there is some indication that pathological liars believe their own lies to the extent of delusion. The claim remains controversial among psychiatrists. Pathological lying is also called mythomania or pseudologia fantastica.
The article states that the extent to which pathological liars appear to believe their own lies is at least enough to cause the pathological liar's behavior or statements to appear delusional to others even if the lying is willful. It is not precisely known whether or not pathological liars can control their lies. In pathological liars, lies do not seem motivated by any obvious external reason for lying. The lying is impulsive, and any psychological reasoning behind the behavior is difficult to determine.