We present experimental evidence of relativistic fine structure in inelastic electron diffraction patterns of a crystalline solid at zone-axis orientation. Diffraction patterns formed by 200 keV electrons suffering energy losses within 10 eV of the silicon K-ionization edge at zone axis exhibit excellent agreement with a relativistic theory of atomic ionization. Such agreement provides experimental confirmation of the theoretical approach, used in an earlier investigation, suggesting that relativistic effects should be taken into account when performing quantitative core-loss spectroscopy on individual atomic columns in the scanning transmission electron microscope.