NEFL and Alzheimer disease: Specifically, we tested the hypotheses that (i) combinations of T‐tau, Ng, and NFL increase the diagnostic accuracy for AD versus controls (CN) and for progressive mild cognitive impairment (PMCI) versus stable MCI (SMCI); (ii) T‐tau, Ng, and NFL have different associations with Aβ pathology and with different clinical stages of AD; and (iii) T‐tau, Ng, and NFL have different associations with other AD features, including cognitive decline, brain atrophy, brain hypometabolism, and white matter hyperintensities (WMH).