A spring mattress is described, including a plurality of strings interconnected side by side. Each string includes a plurality of continuous casings and, enclosed therein, coil springs. Moreover, between at least some of the springs in at least one of the strings a slot is arranged which opens towards the upper or lower side of the mattress. It is preferred for the springs between which the slot is arranged to be separated with an intermediate separation distance, which separation distance exceeds about 10% of the diameter of the largest turn of the adjoining springs. In addition, it is preferred for the casing portions to be arranged at the ends of the spring to be moved towards each other, through the spring, and connected to each other by at least one connecting device.