We examine the theoretical line spectra from hydrogenlike argon in the electron-number-density range of 10 -10 cm and at a temperature corresponding to kT=800 eV. At these high densities it is important to address the relevance of higher-order plasma-ion microfield efFects on the spectral line shape. We begin by calculating spectral line shapes using electric-field-dependent matrix elements that are constructed from wave functions which are solutions of Schrodinger's equation for hydrogenic ions in a uniform field. This provides a suitable basis for including microfield spatial nonuniformities by considering the quadrupole term in the radiator-perturbing ion interaction. Field gradients are calculated in an approximation that takes ion-ion correlations into account. We examine the consequences of these effects as well as of mixing between the adjacent Stark manifold on the Lyman-a and Lyman-P lines of hydrogenlike argon. PACS number(s): 52.25.Nr, 32.70.Jz, 32.30.Rj (APEX) model of the plasma-ion microfield [15,16] thus also incorporating correlations among perturbing ions. This represents an improvement with respect to a previous treatment that only included correlations between the radiator and perturbing ions [6].The power emitted by dipole radiation from an emitter immersed in a plasma, as a function of the frequency, is given by [1,17]