We demonstrate that completely dense yttria-stabilized zirconia (YSZ) electrolyte films <5 mu m thick can be reactively sputter deposited onto porous (similar to 0.5 mu m pore size) La0.8Sr0.2MnO3 substrate/electrodes. The two key factors in achieving the fully dense films were porous substrate preparation and the use of substrate bias during YSZ film deposition. Increasing the negative de substrate bias V-s from 0 to 300 V produced increasingly dense and flat-surfaced films : V-s = 75 V was chosen to yield high density without excessive film compressive stresses. Solid oxide fuel cells consisting of sputtered YSZ films (V-s = 75 V) and a Ni-YSZ fuel electrode on LSM exhibited open-circuit voltage values within 5% of the theoretical values for thicknesses down to 2 mu m. Cells deposited with V-s = 0 V short-circuited at thicknesses of 12 mu m or less.