  Surprisingly, until recently CP violation in pion decay was not ruled out
experimentally even at the percent level. I have derived the first experimental
limit, $-0.01<A_{CP}<0.02$, from old data on the anomalous magnetic moment
$g-2$ of the muon. New data from the Brookhaven $g-2$ experiment might extend
the search by a few orders of magnitude, but probably will not probe the
theoretically-interesting sub-$10^{-7}$ regime.
