The 4s-1s two-quantum decay in the hydrogen atom is calculated in the framework of quantum electrodynamics with allowance for cascades. It is shown that the pure two-photon contribution cannot be separated from the contribution of cascade processes. Interference between the transitions of these two types is comparable in magnitude with the oure two-photon contribution. The separation of pure two-photon contributions is discussed recently in connection with the role of two-photon decays in the separation of radiation from amtter in the early Universe.