You can find the information in the Diagnostic Summary file, which is located on the Study Data download page in the 1st section. In ADNI-1, this information is contained in DXCONV. In ADNI-GO/2, it is in DXCHANGE. Alternatively, you can download the ADNIMERGE file, which is "Key ADNI tables merged into one table" on the download page under Study Info, Data & Databases.