From the analysis of sea-level data near the centre of the former Fennoscandian ice sheet and the theoretical predictions of vertically and laterally stratified Earth models, we explore the extent to which lateral viscosity variations could have influenced the estimate of long term mantle viscosity. We follow a finite element scheme in cylindrical symmetry, focusing on the effects of lateral viscosity contrasts of different magnitudes and wavelengths. Sea-levels are consistent with long wavelength lateral viscosity contrasts of at most 2 orders of magnitude; short wavelength variations, comparable with the lateral extension of the surface load, should be ruled out. Differences in the average viscosities inferred from laterally stratified and uniform mantle models, may be as high as 1 order of magnitude. If we allow for different lateral viscosity patterns in the upper and lower mantle, we find that sea-levels are consistent with short wavelength variations in the upper mantle of around 2 orders of magnitude, coupled with smoother lateral viscosity contrasts in the lower mantle. ¿ American Geophysical Union 1989