This paper summarizes the relevant results of the design, construction, testing, and implementation of a nominal 120 kN magnetorheological damper developed to control a free-plan tall building in Santiago, Chile, equipped with two 160-ton tuned masses. Cyclic as well as hybrid simulation tests were performed on the prototype damper. Global building responses using measured MR properties showed good correlation with analytical estimations. Also, a proposed physical controller for the MR damper was validated through hybrid and building pull-back tests. Its performance is essentially equivalent to that of an LQR controller, but the information needed in its implementation is considerably less. Pull-back tests of 10 cm amplitude were performed on one mass along the flexible edge of the building and its response controlled using the passive and controlled modes of the MR damper. The MR damper was capable of controlling the TM displacements very effectively, as well as the simulated building response for different ground motions and harmonic excitation. Copyright (C) 2010 John Wiley & Sons, Ltd.