This paper describes the performance of a 56,000-m2 office building emphasizing the use of daylighting for ambient illumination. Natural light serves 3,000 employees in open-plan offices through the building's five floors. The architectural scheme includes ceilings that slope from 4.25 m (perimeter) to 2.75 m (center), 3.5-m-deep light shelves at the exterior walls, and a central atrium providing light to interior spaces. An electric lighting system supplements available daylight, when necessary, using fluorescent fixtures with continuously dimming ballasts controlled by photocells. Monitoring during a one-year period has confirmed that the daylighting elements of the building provide ambient illumination in a pattern predicted by the design studies. Between 8AM and 6PM on an average summer day, the buildings southern half can potentially maintain the target illuminance of 350 lux with an electric lighting load of 44% full power. The northern half of the building would require less electrical lighting at 31% of full power. However, actual measured electrical power consumption for ambient lighting is higher at 75% of full power for the south side and 50% of full power for the north side. The daylighing component of interior illuminance peaks at seven times the target level for ambient light. Proper design and tuning of the electric light control system was determined essential for the realization of projected savings in electric power consumption.