We report on the performance of large area NbN nanowire superconducting single-photon detectors (SSPDs). 20×20 µ m2 area SSPDs with 80 and 100 nm linewidths and 50% fill factor were fabricated in 4-nm -thick NbN films grown on single-crystal MgO substrates. The high quality of the devices was verified by electrical and optical testing and compares favorably to measurements of 10×10 µ m2 area SSPDs. Measurements of kinetic inductance versus bias current indicate that the constriction density is low. The fiber-coupled detection efficiency of the devices was 0.4%-3.5% at 100 Hz dark count rate. © 2008 American Institute of Physics.