Metabolic associated fatty liver diseases (MAFLD) definition was proposed to identify fatty liver condition associated to metabolic disorders and to replace non-alcoholic fatty liver disease (NAFLD). We aimed to explore the effect of the application of the new MAFLD criteria on a pre-existing cohort of NAFLD patients. The consequences of the reclassification were investigated by applying the MAFLD criteria to a prospective cohort (The Plinio Study) of dysmetabolic patients examined for the presence of NAFLD. In the Plinio cohort, 795 patients had NAFLD and 767 of them (96.5%) were reclassified as MAFLD patients. Out of these, 94.9% had overweight/obesity or diabetes, while the remaining were lean and had metabolic dysregulation defined by the presence of at least two metabolic risk abnormalities. By contrast, 3.5% of the NAFLD patients were reclassified as no-MAFLD due to the absence of overweight/obesity, diabetes, or metabolic risk abnormalities. The only significant difference between the NAFLD and MAFLD groups was the higher prevalence of subjects with BMI ≥ 25 kg/m2 in the latter (88.6% vs. 92%; p = 0.018). In the cohort, 68 subjects were defined as “lean NAFLD”. Of these, 40 were reclassified as MAFLD and 28 as no-MAFLD. In conclusion, when applying MAFLD criteria to the Plinio cohort, there is a substantial overlap between NAFLD and MAFLD diagnosis. However, some specific subgroups of patients, such as those currently defined as lean NAFLD, were excluded by the new MAFLD definition.