The invention relates to a filter bellows (12), in particular for an air filter system, made of a filter medium (14) which is folded in a zigzag shape. Respective folding edges (26) extend between opposite end edge surfaces (20a, 20b) formed from end edges (22a, 22b), and a respective end fold (24) with a folding edge (26) and an opposite free end edge (30) is arranged on opposite end faces (36, 38). Folding edges (26) lie on a surface (44) which is on the crude fluid side in the installed use state and on an opposite surface (46) which is on the purified fluid side in the installed use state. A passage (50) is formed between the crude fluid-side surface (44) and the purified fluid-side surface (46), said passage having a crude fluid-side opening (60) with a crude fluid-side cross-section (54) and a purified fluid-side opening (62) with a purified fluid-side cross-section (56). In a projection of the openings (60, 62) towards each other, one of the cross-sections (54, 56) is arranged outside of the other cross-section (56, 54) at least in some regions. The invention further relates to a filter element (10) comprising such a filter bellows (12), to the use of the filter element (10) as a flat air filter, in particular as a flat air filter of an internal combustion engine, and to a filter system (100) comprising such a replaceable filter element (10).