Abstract: In a filter element comprising a pleated pack of planar porous filter material sheets with pleat backs, which are reinforced by impregnation or coating with a casting resin or another self-hardening plastic material, the filter element includes transition areas between the impregnated or coated and the non-impregnated or non-coated filter areas wherein the hardened state of the filter material gradually changes to the non-hardened state thereby to reduce chances of fatigue fractures between the hardened and the non-hardened filter areas.