The internal luminescence quantum efficiency q is one limiting factor for the performance of organic light-emitting diodes. Photoluminescence measurements are frequently used to estimate q but these neglect effects of the local environment of the emissive sites and the electrical excitation mechanism. We present a method for the in situ measurement of q under electrical operation. The current efficiency of several devices with different emitter-cathode distances is quantitatively compared. Precise optical simulation allows determining q without additional assumptions.