Functional MRI (fMRI) in established Alzheimer's disease (AD) shows regionally altered blood oxygenation level dependent (BOLD) responses. Mild cognitive impairment (MCI) is thought to represent an intermediate state between health and early Alzheimer's disease. To study this probable early dementia stage pathology, we studied in detail the BOLD response in MCI during visual encoding. 28 MCI patients, 18 AD patients, and 41 healthy elderly controls performed a face encoding task during fMRI scanning. Data were analyzed using orthogonal regressors, each representing different phases of the BOLD response (from slow to fast). Using a mixed effects model, regressor x group interactions were analyzed applying P < 0.05, corrected. In occipital regions, MCI patients could be distinguished significantly better from controls and AD patients with a regressor of the early phase of the (fast) BOLD response than with the regressor of the late (slow) BOLD phase. Occipitally, the early phase BOLD response was significantly diminished in MCI patients compared to controls, and significantly increased when compared to AD. AD patients showed diminished early phase activation in widespread regions throughout the brain when compared to controls. There were no differences in the late (slow) phase of the BOLD response. This study stresses the importance of analyzing early phase BOLD responses and not only using one model of the BOLD response in neurodegenerative diseases. The increasing delay of the BOLD response from controls to MCI to AD may be consistent with the idea that MCI is a transitional state between healthy aging and dementia. Analyzing differences in different phases of the BOLD response introduces new opportunities to understand changes in regional brain dynamics in MCI and how well this may serve as an early marker of AD pathology.