We present a biosensor system comprising a chip with an array of silicon nanowire ISFETs and a CMOS chip with the signal-conditioning circuitry. An array of 48 silicon nanowires has been patterned via e-beam lithography. The on-chip circuitry is used to monitor the resistance of the nanowires by applying a constant voltage and measuring the respective current. The analog signal is then digitized on chip and transmitted. The system has been successfully fabricated and tested. Measurements showing the advantage of using CMOS readout are presented: 1) simultaneous readout of different silicon nanowires, 2) low noise and 3) fast measurements.