This paper deals with stiffness control of orthopaedic mattresses. There are several pneumatic elements inserted into the body of the mattress. Pressure transducers measure pressure in the elements. Pneumatic valves control input and output air flow. All components including a compressor are inserted in the bed. The initial control algorithm was designed in LabVIEW. The final solution is realized through a microprocessor. It is possible to set the desired stiffness in two zones of the mattress - in the shoulder area and in the hip area. The system has two modes. The desired values can be adjusted manually in order to achieve level position of the spine. In the second case, the stiffness can be changed dynamically and may act against bedsore.