Mild cognitive impairment (MCI) can be detected at any time in subjects with Parkinson’s disease (PD) and less frequently it can be present one of the symptoms of disease onset. A deep investigation of motor and non-motor symptoms, which might be overlooked or absent at the earliest stages, and the use of diagnostic tools including molecular imaging can help to disclose typical features suggesting that MCI is due to PD or, more generally to a Lewy-body disease (LBD).