An experimental approach to the determination of the band-gap renormalization caused by many-body effects in quantum wells (QW s) at room temperature is presented. The chemical potential and the corresponding plasma density are derived from independent measurements of the junction voltage and the differential carrier lifetime in GaAs/AlxGa1-xAs single-quantum-well (laser) diodes as a function of the carrier-injection rate. Within a plasma-density range from 3×1011 to 7×1012 cm-2, the renormalization of the chemical potential is determined from the difference between the measured chemical potential and the calculated noninteracting chemical potential that would be observed in the absence of a band-gap shrinkage. If the occupation of higher subbands is negligible, this difference equals the renormalization of the fundamental band gap. Corresponding requirements to the design of QW structures are discussed. The experimentally determined renormalization of the chemical potential is compared to results of a detailed theoretical calculation of band-gap renormalization, taking into account the self-energy shifts of all subbands of the complex QW device. © 1992 The American Physical Society.