Expressions for direct correlation function, C(Q), and structure factor, S(Q), of liquid alkali metals (Na, Rb, Cs) and simple dense fluids (Ar, Kr) have been derived using the linear isotherm regularity (LIR). Unlike previous models, it is shown in this work that C(Q) and, consequently, S(Q) can be obtained without employing any specific potential; only p-v-T experimental data have been used for the calculation of C(Q) and S(Q). With these expressions, it is possible to determine the height and position of the first peak of S(Q) with an acceptable accuracy. The limiting of S(Q) at Q = 0 is calculated and compared with the values obtained experimentally, and those calculated from the extended random phase approximation (ERPA) and random phase approximation (RPA) models. We have used the derived expressions for S(Q) of liquid alkali metals and simple dense fluids for prediction of the pressure and density derivatives of S(Q) in the vicinity of the first peak. (c) 2005 Elsevier B.V. All rights reserved.