  We calculate the first vertex correction $\delta\Lambda_{\bf k}$ to the bare
vertex $\Lambda_\circ$ in the nearly antiferromagnetic spin fluctuation Fermi
liquid theory of the cuprate superconductor ${\rm YBa_2Cu_3O_7}$. It is
calculated for $\bf k$ on the Fermi surface, and ${\bf Q}=(\pm {\pi\over a},
\pm {\pi\over a})$. We find that the dimensionless ratio $|\delta\Lambda_{\bf
k}|/\Lambda_\circ$, which parametrizes the vertex correction, is not small. It
is a maximum for ${\bf k=k_h}$, where ${\bf k_h}$ is a ``hot spot'' on the
Fermi surface. This makes large quantitative corrections to the theory.
