Distributed antenna system (DAS) can provide high speed data services for mobile terminals (MTs) within multi-floor buildings by reducing the radio transmission distance between the transmitter and the receiver. In this paper, the uplink spectral efficiency of an in-building DAS with frequency reuse is studied, where remote antenna units (RAUs) are deployed on each floor throughout the building and connected to a central unit (CU) where received signals are processed. The proposed scheme exploits the penetration loss of the signal through the floors, resulting in frequency reuse in spatially separated floors, which increases system spectral efficiency and also reduces co-channel interference. The impact of co-channel interference on system performance is investigated using a propagation channel model derived from multi-floor in-building measurement results. System performance is evaluated in terms of location-specific spectral efficiency for a range of potential mobile terminal (MT) locations and various in-building propagation characteristics. Numerical results obtained suggest that the proposed scheme can facilitate better use of the available radio spectrum, and provide higher data rates for indoor MTs.