The Fisher-Shannon complexity is used to quantitatively estimate the contribution of relativistic effects to on the internal disorder of Klein-Gordon single-particle Coulomb systems which is manifest in the rich variety of three-dimensional geometries of its corresponding quantum-mechanical probability density. It is observed that, contrary to the non-relativistic case, the Fisher-Shannon complexity of these relativistic systems does depend on the potential strength (nuclear charge). This is numerically illustrated for pionic atoms. Moreover, its variation with the quantum numbers (n, l, m) is analysed in various ground and excited states. It is found that the relativistic effects enhance when n and/or l are decreasing. PACS numbers: 89.70.Cf, 03.65.Pm Numerous phenomena and properties of manyelectron systems have been qualitatively characterized by information-theoretic means. In particular, various single and composite information-theoretic measures have been proposed to identify and analyze the multiple facets of the internal disorder of non-relativistic quantum systems; see e.g. Ref. [1][2][3][4][5][6][7][8][9].They are often expressed as products of two quantities of local (e. g. the Fisher information) and/or global (e. g. the variance or Heisenberg measure, the Shannon entropy, the Renyi and Tsallis entropies and the disequilibrium or linear entropy ρ ) character, which describe the charge spreading of the system in a complementary and more complete manner than their individual components. This is the case of the disequilibrium-Shannon or Lopez-Ruiz-Mancini-Calvet (LMC in short) [2], disequilibrium-Heisenberg [4], Fisher-Shannon [5,8] and the Cramer-Rao [5, 10] complexities, which have their minimal values at the extreme ordered and disordered limits.Recently these studies have been extended to take into account the relativistic effects in atomic physics. Relativistic quantum mechanics [11] tells us that special relativity provokes (at times, severe) spatial modifications of the electron density of many-electron systems, what produces fundamental and measurable changes in their physical properties. The qualitative and quantitative evaluation of the relativistic modification of the spatial redistribution of the electron density of ground and excited states in atomic and molecular systems by informationtheoretic means is a widely open field. In the last three years the relativistic effects of various single and composite information-theoretic quantities of the ground states of hydrogenic [12] and neutral atoms [13][14][15] have been investigated in different relativistic settings.First Borgoo et al [13] (see also [16]), in a Dirac-Fock setting, find that the LMC shape complexity of the * Electronic address: manzano@ugr.es † Electronic address: slopez@ugr.es ‡ Electronic address: dehesa@ugr.es ground-state atoms (i) has an increasing dependence on the nuclear charge (also observed by Katriel and Sen [12] in Dirac ground-state hydrogenic systems), (ii) manifest shell and relativistic effects, the latter b...