This page describes longitudinal data sets, some of them publically available, some for participating investigators.
The Alzheimer's Disease Neuroimaging Initiative
Three stages (ADNI, ADNI-GO and ADNI2) exists with 800-1000 subjects and several time points approx. 6 months to a year apart.
See also the world wide ADNI:
Longitudinal MRI Data in Nondemented and Demented Older Adults
This set consists of a longitudinal collection of 150 subjects aged 60 to 96. Each subject was scanned on two or more visits, separated by at least one year for a total of 373 imaging sessions
Minimal Interval Resonance Imaging in Alzheimer's Disease
The MIRIAD dataset is a database of volumetric MRI brain-scans of 46 Alzheimer's sufferers and 23 healthy elderly people. Many scans were collected of each participant at intervals from 2 weeks to 2 years and includes 708 scans.
Parkinson's Progression Markers Initiative
Efficient Longitudinal Upload of Depression in the Elderly
The ELUDE dataset is a longitudinal study of late-life depression at Duke University. There are 281 depressed subjects and 154 controls included. An MR scan of each subject was obtained every 2 years for up to 8 years (total of 1093 scans).
The Dominantly Inherited Alzheimer Network
Autism Brain Imaging Data Exchange
Federal Interagency Traumatic Brain Injury Research
FITBIR may have some longitudinal data (after obtaining an account).
CCBD One-month Test-Retest Reliability and Dynamical Resting-State Study
30 subjects scanned 10 times within a month (anatomical, resting state, DTI, ASL). Center for Cognition and Brain Disorders (CCBD) at Hangzhou Normal University (HNU) http://fcon_1000.projects.nitrc.org/indi/CoRR/html/hnu_1.html