Strain relaxation and the ferroelectric critical temperature were investigated in a series of epitaxial Pb(Zr0.20Ti0.80)O-3 thin films of different thicknesses grown on metallic 0.5% Nb-doped SrTiO3 substrates. Detailed x-ray diffraction studies reveal that strain relaxation progressively occurs via misfit dislocations as the film thickness is increased from fully coherent films (for films below 150 A) to essentially relaxed films (for thicknesses above typically 800 A). It is found that this change in the strain state does not modify the ferroelectric critical temperature which is found for all the samples to be around 680 degrees C, a value much higher than the bulk.