What are the tax implications of a partial wash sale?

A partial wash sale occurs when you sell some, but not all, of a security at a loss and then repurchase the same or substantially identical security within 30 days. The tax implications of a partial wash sale are that you cannot claim the loss on the portion of the security that was repurchased. This means that the disallowed loss is added to the cost basis of the repurchased security, which can affect your future capital gains taxes.