We present the result of a combined ab initio theoretical study and ultraviolet photoemission spectroscopy experiment of the electronic surface states of the clean and of the oxygen covered, unreconstructed and missing-row reconstructed, Ag(001) surface. For the clean and for the unreconstructed oxygen covered surface, theory and experiment find an electronic surface state at about 0.4 eV below the Fermi energy (EF) at the B point of the surface Brillouin zone. At the same point, theory predicts the existence of an empty state at +3 eV. Upon reconstruction, the photoemission signal of the filled state is strongly reduced and, according to calculations, the state shifts in energy above EF The surface interband transition (SIT) between the two states exists thus only for the unreconstructed surfaces and nearly matches the surface plasmon energy. Our investigation confirms therefore the correlation between the presence of this SIT and the anomaly of the surface plasmon dispersion curve of unreconstructed Ag(001). (C) 2001 Elsevier Science B.V. All rights reserved.