In the current context of a world energy situation that demands high-quality indoor environments compatible with rational energy use, the purpose of this work is to present a successful case study of a comfortable low-energy office building in Madrid, Spain. An exhaustive indoor air quality and thermal comfort field measurement survey was carried out during the summer period. The main global and local thermal comfort parameters in several workplaces, as well as CO 2 concentration in supply air, exhaust air, and occupied space, were registered. Correct operation of the demand-controlled ventilation system and air distribution is observed. Also, thermally activated building systems and shading design make it possible to get high levels of indoor thermal comfort, as derived from measurements. This proves that the integration of novel energy-efficient concepts in office building design can not only contribute to a minimization of the energy use but can simultaneously favor the achievement of high indoor environmental quality.