Crucial Role of the Lattice Distortion in the Magnetism of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>LaMnO</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

The stability of the $A$-type antiferromagnetic order and canted magnetic structure of LaMn${\mathrm{O}}_{3}$ perovskite is explained in the itinerant-electron picture based on the local-spin-density approximation. We demonstrate the crucial role of the observed lattice distortion which strongly affects the magnetocrystalline anisotropy, as well as the anisotropic and isotropic exchange interactions in this compound.