Describes research into the cost of aircraft structural modifications. Detailed cost and man-hour data supplied by the airframe industry permitted derivation of estimating tools for major aircraft components. Separate equations were derived for engineering, tooling, manufacturing, and material cost. The major explanatory variable was always weight. It was hypothesized that structural modification cost could be estimated on the basis of the weight of material added. Estimates of the cost of modification for the B-52, C-141, C-5, and EF-111 were compared with cost data from industry. Considerable informed judgment is required as is a knowledge of such program-specific facts as whether the original production tooling still exists. Rather than a mathematical model, the study describes the kinds of information needed, suggests guidelines, and presents estimating equations for airframe systems and subassemblies. These contribute to an understanding of the estimating problem but do not constitute a general solution.