Thor Power Tool Company v. Commissioner, 439 U.S. 522 (1979), was a United States Supreme Court case in which the Court upheld IRS regulations limiting how taxpayers could write down inventory.

But the Court upheld the IRS regulations, saying, "There is no presumption that an inventory practice conformable to 'generally accepted accounting principles' is valid for tax purposes.

Such a presumption is insupportable in light of the statute, this Court's past decisions, and the differing objectives of tax and financial accounting."

The Thor decision caused publishers and booksellers to be much quicker to destroy stocks of poorly-selling books in order to realize a taxable loss.

These books would previously have been kept in stock but written down to reflect the fact that not all of them were expected to sell.