If Sparrow gets stuck loading, try this: (1) Uninstall Sparrow. (2) Open a terminal window. (3) Delete these files: ~/Library/Preferences/com.sparrowmailapp.sparrowappcast.plist ~/Library/Application Support/Sparrow (with all its subfolders) (4) Reinstall Sparrow. Worked for me. My problem was apparently due to my having created custom labels that Sparrow must not have liked. My message base was not altered by deleting these files, since I use Gmail IMAP, synchronizing my inbox and other email folders with Gmail's web server (or at least I think that's how it works). If you don't use IMAP or otherwise have a local message database, you may need to make a backup first.