[en] In a Λ cold dark matter (ΛCDM) cosmology, the baryonic Tully–Fisher relation (BTFR) is expected to show significant intrinsic scatter resulting from the mass–concentration relation of dark matter halos and the baryonic-to-halo mass ratio. We study the BTFR using a sample of 118 disk galaxies (spirals and irregulars) with data of the highest quality: extended $\mathrm{H}\mathrm{I}$ rotation curves (tracing the outer velocity) and Spitzer photometry at 3.6 μm (tracing the stellar mass). Assuming that the stellar mass-to-light ratio (${\mathrm{{\rm Y}<!--\; ??\; -->}}_{\ast <!--\; ???\; -->}$) is nearly constant at 3.6 μm, we find that the scatter, slope, and normalization of the BTFR systematically vary with the adopted ${\mathrm{{\rm Y}<!--\; ??\; -->}}_{\ast <!--\; ???\; -->}$. The observed scatter is minimized for ${\mathrm{{\rm Y}<!--\; ??\; -->}}_{\ast <!--\; ???\; -->}\gtrsim <!--\; ???\; -->0.5$ $M_{\odot <!--\; ???\; -->}/L_{\odot <!--\; ???\; -->}$, corresponding to nearly maximal disks in high-surface-brightness galaxies and BTFR slopes close to ∼4. For any reasonable value of ${\mathrm{{\rm Y}<!--\; ??\; -->}}_{\ast <!--\; ???\; -->}$, the intrinsic scatter is ∼0.1 dex, below general ΛCDM expectations. The residuals show no correlations with galaxy structural parameters (radius or surface brightness), contrary to the predictions from some semi-analytic models of galaxy formation. These are fundamental issues for ΛCDM cosmology.