Multi-photon nonlinear laser wave-mixing spectroscopy is presented as an ultrasensitive detection method for pancreatic cancer biomarkers, carbohydrate antigen 19-9 (CA 19-9) and carbohydrate antigen 242 (CA 242), and heart-failure biomarkers, pro-atrial natriuretic peptide (proANP) and brain natriuretic peptide (BNP). Wave mixing is an ultrasensitive optical absorption-based detection method, and hence, it can detect both fluorescing and non-fluorescing biomarkers. One can detect biomarkers in their native form, label-free, without the use of time-consuming labeling steps. The wave-mixing signal beam is strong, collimated and coherent (laser-like) and it can be collected using a simple photodetector with an excellent signal-to-noise ratio (S/N). The wave-mixing signal has a quadratic dependence on the analyte concentration, and thus, small changes in analytes can be monitored more effectively (i.e., an ideal sensor). Compared to currently available detection methods, wave mixing offers inherent advantages such as short optical path length (micrometer-thin samples), high spatial resolution, ultrasensitive detection limits comparable or better than those of fluorescence- or ELISAbased methods, and native label-free detection. Since laser wave-mixing probe volume is small (nanoliter to picoliter), it is intrinsically convenient to couple with microfluidics or capillary-based electrophoresis systems to enhance chemical specificity. Different biomarkers can be placed on a simple slide or flowed inside a capillary and then detected. Our nonlinear multi-photon detectors can be easily configured as portable battery-powered devices that are suitable for use in the field. Potential real-world applications include detection of various biomarkers, cancer cells, and reliable early detection of diseases.