A general theoretical model of the origin of the magnetic anisotropy in paramagnetic metal-containing liquid crystals is developed. General relations between the molecular magnetic anisotropy of mesogenic lanthanide complexes and the macroscopic magnetic anisotropy of these liquid crystals in the mesophase are obtained. The net magnetic anisotropy of a real metallomesogen is shown to be the result of a complex interplay between the molecular magnetic anisotropy, orientation of the long molecular axis, and disorder effects. The sign of the magnetic anisotropy Delta chi depends not only on the anisotropy of the tensor of molecular magnetic susceptibility, but also on the orientation of the long molecular axis of rodlike lanthanide complexes with respect to the principal magnetic axes of the molecular tensor of magnetic susceptibility. The influence of micro- and macroscopic disorder in real liquid crystals is discussed. Numerical parametric calculations were used to rationalize the variation of the magnitude and sign of the magnetic anisotropy in a series of isostructural lanthanide-containing metallomesogens. Experimental magnetic susceptibility and magnetic anisotropy of a series of [Ln(LH)(3)(NO3)(3)] compounds (LH is a Schiff base) are well reproduced by calculations based on the present model. Limitations of the Bleaney theory of magnetic anisotropy are analyzed.