Twelve steam generators in three nuclear power plants were damaged during manufacture by postweld stress relief heat treatment. This damage was detected after all were installed, but before any of the plants became operational. The steam generators were rebuilt, in situ, by complete replacement of the internals and tubing. The damage caused by heat treatment consisted of indentation of the tubing by distortion of the tube support plates. The indentations produced points of local high stress in the tubing, causing a potential for stress corrosion cracking. The indentations and the distortion of the support plates also prevented free axial motion of the tubing through the support plates in service, and it is thought that some tubes would have failed in service for this reason. The rebuilding program permitted design changes to accommodate technical information accumulated since the vessels were designed in 1975. These design changes principally were a strengthening of internal structural members to resist seismic forces and fluid forces predicted for pipe breaks, substitution of stainless steel for carbon steel tube support plates, a more elastic U-bend restraint design, increase of some clearances to better tolerate in-service thermal distortion, and a redesigned emergency water injection system to promote thermal syphoning of the reactor coolant under accident conditions. New methods of tube expansion, tube-to-tube-sheet welding, and heat treatment of the tubing were developed to facilitate field installation. The work required 15 months. It was planned and executed so that the delays to the erection programs of the plants were minimized. The final result was a significant improvement in the design of the steam generators, and an accumulation of technology that may be useful if steam generators should require in-service retubing.