  Aboav's law is a quantitative expression of the empirical fact that in planar
cellular structures many-sided cells tend to have few-sided neighbors. This law
is nonetheless violated in the most widely used model system, {\it viz.} the
Poisson-Voronoi tessellation. We obtain the correct law for this model: Given
an $n$-sided cell, any of its neighbors has on average $m\_n$ sides where
$m\_n=4+3(\pi/n)^{-{1/2}}+...$ in the limit of large $n$. This expression is
quite accurate also for nonasymptotic $n$ and we discuss its implications for
the analysis of experimental data.
