Subtraction radiography permits the use of 7-10 times less contrast material than is required without subtraction techniques. The first-order, second-order, and composite mask techniques are described and evaluated. While the second-order technique gives more complete cancellation of common structures, the more commonly used first-order technique was found to be adequate for most clinical uses. The smallest change in contrast which can be detected using subtraction radiography is limited by inherent background film noise in the original radiographs and not by statistical fluctuations in the x-ray quanta.