If you sell and buy back within 30 days you are subject to the 'wash rules'. This basically means that you cannot take the loss on next year's return. Your basis will simply be adjusted. The strategy some people do is sell in one account and buy in another. This way if it goes up substantially you don't lose out on the runup and you are able to take the loss on next year's return. Also keep in mind that even if you lose 25k overall during any particular calendar year, IRS only permits you to take a 3k loss. You can carryforward the loss to future years, and offset any future gains against the loss.