  We study the thermodynamics of the electroweak theory at a finite lepton
number density. The phase diagram of the theory is calculated by relating the
full 4-dimensional theory to a 3-dimensional effective theory which has been
previously solved using nonperturbative methods. It is seen that the critical
temperature increases and the value of the Higgs boson mass at which the first
order phase transition line ends decreases with increasing leptonic chemical
potential.
