We identify a quantum well internal high density Augerlike loss process as the origin of the so called ‘droop’ of internal quantum efficiency (IQE) in InGaN based light emitters. The IQE of such a device peaks at small current densities and then monotonously decreases towards higher currents. The origin of this ‘droop’ has been widely discussed recently and many possible mechanisms have been proposed for explaining the effect. We compare temperature and carrier density dependent electroluminescence and photoluminescence measurements of a green emitting single quantum-well (SQW) LED over a wide parameter range. The carrier-density as well as temperature dependence of efficiency is identical in both measurements, indicating that the decrease is due to a high density quantum-well internal loss process. The data can be accurately modeled assuming an Auger-like loss process with C = 3.5 × 10–31 cm6s–1. We suggest phonon- or defect-assisted Auger recombination as the origin of this loss-channel. The high current performance can be improved if a thick InGaN SQW or a multi quantum-well (MQW) is used. This is in very good agreement with theoretical simulations (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)