Originally Posted by SmokeyTheBear

http://lifehacker.com/396928/mark-al...-read-in-gmail

Create a new filter with is:unread in the Has the words field, then tell Gmail to Mark as read all matching messages. Finally, tick the box next to Also apply filter to conversations below and create the filter. The filter will mark all your unread items as read. If you just want to clear out unread items in a specific label, add something like label:Followup to the Has the words box with is:unread.