A filter element for fluids, in particular for hydraulic fluid, has a foldable filter casing (10) with at least one filter layer (18, 20, 22, 24) extending between two end caps (26, 32). To vary in regions the thickness of the filter casing (10), the height (h1) of each filter fold (12, 44) increases from one end cap (26) to the other end cap (32). Alternatively, with the filter fold height (h1) being maintained, the outer diameter (40) of the filter casing (10) varies in the direction of one of the end caps (26, 32).