This cross‐sectional study presents several lines of evidence suggesting that MTL tau pathology plays an important role in BF cholinergic degeneration in preclinical and prodromal stages of AD: (1) The effect of Aβ burden on Ch4 volume was significant in CU MTL+ individuals but not in CU MTL− individuals, highlighting the concurrence of Aβ and MTL tau pathology in association with early Ch4 volume loss. Here, MAPT is linked to Alzheimer disease.