The Navier-Stokes system for heat-conducting incompressible fluids is studied in a domain ${Omega}{subset}R^3$. The viscosity, heat conduction coefficients and specific heat at constant volume are allowed to depend smoothly on density and temperature. We prove local existence of the unique strong solution, provided the initial data satisfy a natural compatibility condition. For the strong regularity, we do not assume the positivity of initial density; it may vanish in an open subset (vacuum) of ${Omega}$ or decay at infinity when ${Omega}$ is unbounded.