An experimental long-range lightning detection system consisting of a network (ZEUS) of six ground-based radio receivers has been deployed in Europe and has been operated since June 2001. The receivers detect and measure the electromagnetic signal emitted by a lightning source in the Very Low Frequency band, between 5 and 15 kHz (sferics), which propagates over thousands of kilometers in the Earth-ionosphere waveguide. In this study, lightning location retrievals from the ZEUS network are compared against more definitive sources in order to investigate issues on long-range lightning retrieval accuracy and detection efficiency. The validation study is carried out over three regions: the U.S. East Coast/Northwestern Atlantic, the African continent, and within the network (Spain). Data originate from the U.S. National Lightning Detection Network, the Lighting Imaging Sensor aboard the Tropical Rainfall Measurement Mission Satellite, and the Spanish National Lightning Network. We investigate the nature of the errors involved in the lightning location retrieval, and propose a new approach for moderating some of these effects. Location errors are shown to vary between 40 and 400 km (mode at 220 km) for very long ranges (>5000 km). Within the network, the distance error does not exceed 40 km (mode at 20 km). Random simulation results are shown to be in good agreement with validation data. Error reduction of 12% (or 20 km at long-range) can be achieved by accommodating for the varying propagation velocities in the sferics wave. The long-range detection efficiency is shown to have a mode at approximately 20% and a relatively small spread with no obvious deviation from day to night.