Arrays of cross-reactive sensors are well suited to detect subtle changes in clinical samples resulting from internal diseases. Monolayer-capped metallic nanoparticles (MCNPs) are ideal base materials for such sensor arrays because they are chemically versatile, easily fabricated, and readily integrated into chemiresistive sensing platforms using microelectrodes. In this review, we present the main concepts and approaches related to the use of MCNPs in chemiresistors and show their applicability to exhaled breath testing, which is a new non-invasive frontier of medical diagnostics.