We present a theoretical study on how many-body effects, especially the exchange interaction, can affect the spin-splitting of a two-dimensional electron gas (2DEG) in the presence of the Rashba spin–orbit interaction. The standard Hartree–Fock approximation and Green's function approach are employed to calculate self-consistently the energy spectrum and density of states of a spin-split 2DEG realized from an InGaAs/InAlAs heterojunction. We find that although the Hartree interaction affects rather weakly the self-energy of a quasi-particle, the presence of the exchange (Fock) interaction can significantly enhance the spin-splitting of a 2DEG on top of the Rashba effect. The main physical reasons for this important phenomenon are examined and discussed.