We present the fabrication, characterization and successful medical application of a membrane-type surface stress sensor (MSS), arranged in arrays for molecular detection in gaseous phase. Made out of SOI substrate, a round membrane with a diameter of 5 0 0 mu m and a thickness of 2.5 mu m is suspended by four sensing beams with integrated p-type piezoresistors, composing a full Wheatstone bridge. The membrane is coated with a thin polymer layer, which reacts with volatile molecules and produces a deflection of the membrane. The membranes were functionalized with various polymers and characterized as humidity sensors with a sensitivity of 87 mV/%RH and a time constant (Tau63%) of 1.3 s. Finally, through breath analysis and the use of principal component analysis (PCA), we were able, in a double blind trial, to distinguish cancer patients and healthy persons.