The development of white organic light-emitting diodes1 (OLEDs) holds great promise for the production of highly efficient large-area light sources. High internal quantum efficiencies for the conversion of electrical energy to light have been realized2, 3, 4. Nevertheless, the overall device power efficiencies are still considerably below the 60–70 lumens per watt of fluorescent tubes, which is the current benchmark for novel light sources. Although some reports about highly power-efficient white OLEDs exist5, 6, details about structure and the measurement conditions of these structures have not been fully disclosed: the highest power efficiency reported in the scientific literature is 44 lm W-1 (ref. 7). Here we report an improved OLED structure which reaches fluorescent tube efficiency. By combining a carefully chosen emitter layer with high-refractive-index substrates8, 9, and using a periodic outcoupling structure, we achieve a device power efficiency of 90 lm W-1 at 1,000 candelas per square metre. This efficiency has the potential to be raised to 124 lm W-1 if the light outcoupling can be further improved. Besides approaching internal quantum efficiency values of one, we have also focused on reducing energetic and ohmic losses that occur during electron–photon conversion. We anticipate that our results will be a starting point for further research, leading to white OLEDs having efficiencies beyond 100 lm W-1. This could make white-light OLEDs, with their soft area light and high colour-rendering qualities, the light sources of choice for the future.