This paper presents the high-precision theoretical predictions for $e^+e^- to l^-l^+$ scattering. Calculations are performed using the {tt SANC} system. They take into account complete one-loop electroweak radiative corrections as well as longitudinal polarization of initial beams. Reaction observables are obtained using the helicity amplitude method with taking into account initial and final state fermion masses. Numerical results are given for the center-of-mass energy range $sqrt{s}=250-1000$ GeV with various degrees of polarization.