Theoretical study presented in this paper is related to the general problem of the role of interference effects in the tunneling of electrons in STM systems. The specific aim of this work is to clarify the possible influence of the intra-atomic interference connected with the electron tunneling through s and P-z orbitals of the tip and the sample on the results of scanning tunneling spectroscopy (STS). This kind of interference can increase or reduce the efficiency of the tunneling through s and p(z) orbitals of the STM system. It is shown that this factor may significantly influence the current-voltage dependences and modify considerably the STS features connected with s and p(z) states of the tip and the sample. STS simulations indicate that the shape and origin of important features in the current-voltage dependences and STS spectra may represent the effect of the interorbital interference which cannot be explained by density-of-states distributions of the tip and the sample. (C) 2004 Elsevier B.V. All rights reserved.