Currently, there is not a specific recommendation to combine peak displacements in orthogonal directions for the design of base isolators available in the literature. The 100%+30% combination rule prescribed by many seismic codes worldwide to account for orthogonal effects in the design of conventional structures is sometimes used for base-isolated structures. This combination rule was formerly proposed to account for lateral forces, but it should be studied whether or not this rule still applies for the design displacements of isolated systems. This paper presents a parametric study where it is found that the statistical responses to combine peak displacements in orthogonal directions are not independent from the effective period for both linear and bilinear isolators, so a constant combination rule is somewhat incorrect.