This paper describes the fabrication technique for the realization a microfluidic flow cell to be integrated on a CMOS biosensor for DNA hybridization detection. The main element of the microfluidic system is made in polydimethylsiloxane (PDMS) elastomer and takes up an area of 5 mm(2). PDMS is cast against a silicon master patterned by Deep Reactive Ion Etching (DRIE) and then bonded on the chip by means of oxygen plasma activation. The micro channels patterned in the flow cell are connected with capillary tubes that can be easily interconnected to a common syringe.