  We show that in theories where neutrino masses arise from type I seesaw
formula with three right handed neutrinos and where large atmospheric mixing
angle owes its origin to an approximate leptonic $\mu-\tau$ interchange
symmetry, the primordial lepton asymmetry of the Universe, $\epsilon_l$ can be
expressed in a simple form in terms of low energy neutrino oscillation
parameters as $\epsilon_l = (a \Delta m^2_\odot+ b \Delta m^2_A
\theta^2_{13})$, where $a$ and $b$ are parameters characterizing high scale
physics and are each of order $\leq 10^{-2} $ eV$^{-2}$. We also find that for
the case of two right handed neutrinos, $\epsilon_l \propto \theta^2_{13}$ as a
result of which, the observed value of baryon to photon ratio implies a lower
limit on $\theta_{13}$. For specific choices of the CP phase $\delta$ we find
$\theta_{13}$ is predicted to be between $0.10-0.15$.
