A foldable, intravascularly insertable blood pump (10) comprises a foldable, radially delivering impeller (20) accommodated in a foldable housing or envelope (24). Axially before and behind the impeller a clearance (60) is provided between the impeller (20) and the front and back walls (32, 62) of the housing (24), the clearance being at least 0.2 mm. Preferably, the space defined between adjacent blades (54) of the impeller (20) is open towards the front wall (32) of the housing (24).