Three case histories of large dams built in North Patagonia which experienced unforeseen problems during construction, or after several years of operation are described. The necessary remedial and corrective works involved the development of important programs, being economic and programmatic impacts of great magnitude. The lessons learned from these experiences were very useful to the practice of design and construction of dams in the region. At the sites of three projects: Casa de Piedra, Alicura and El Chocon weak rock foundation are founded. In Casa de Piedra, located in a regional environments with clear evidence of limestone and gypsum formations, the use of microgravimetry was appropriate for the detection of cavities or discontinuities that traditional survey research may not detect. In Alicura where major structures were located on the left abutment, it was important to increase knowledge in the sector through early specific exploratory interventions, such as trenches, deep wells and exploration galleries. The importance of a good drainage system and percolation controls during operation through galleries and drains was fundamental. The case of El Chocon, where the situation becomes critical after ten years of normal operation, again shows the need for control and monitoring of the project throughout the useful life of the dam. The instrumentation system and the permanent control carried out by the Owner, Hidronor, made it possible to detect unfavorable conditions and plan an adequate corrective action in time.