  We study the effect of disorder on the order parameter equation and
transition temperature of a Pomeranchuk-type Fermi-surface instability using
replica mean field theory. We consider the example of a phase transition to a
$d_{x^2 +y^2}$ type Fermi surface distortion, and show that, in the regime
where such a transition is second order, the transition temperature is reduced
by disorder in essentially the same way as that for a d-wave superconductor. We
argue that observing this disorder dependence of metal-to-metal transition
might be a useful indicator of a finite angular momentum Fermi surface
distortion.
