In order to create an environmentally-conscious building, the environmental impacts of the entire service life must be known. Life-cycle assessment (LCA), which evaluates the impacts from all life-cycle phases, from "cradle to grave," is the best method to achieve this goal. In this paper, LCA is used to quantify the energy use and the environmental emissions during the construction phase of two typical office buildings, one with a structural steel frame and one with a cast-in-place concrete frame, and then these are put in the perspective of the overall service life of each building. The concrete structural-frame construction has more associated energy use, CO2, CO, NO2, particulate matter, SO2, and hydrocarbon emissions due to more formwork used, larger transportation impacts due to a larger mass of materials, and longer equipment use due to the longer installation process. In contrast, the steel-frame construction has more volatile organic compound (VOC) and heavy metal (Cr, Ni, Mn) emissions due to the painting, torch cutting, and welding of the steel members. The energy use and the environmental emissions of the two buildings are comparable if the total impacts from materials' manufacturing, construction, transportation, use, maintenance, and demolition are considered. Energy use and environmental emissions from office buildings can be reduced through a careful selection of embedded and temporary materials and construction equipment.