The development and performance of a cw, TE-cooled DFB quantum cascade laser based sensor for quantitative measurements of ammonia (NH 3) and nitric oxide (NO) concentrations present in exhaled breath will be reported. Human breath contains ∼ 500 different chemical species, usually at ultra low concentration levels, which can serve as biomarkers for the identification and monitoring of human diseases or wellness states. By monitoring NH 3 concentration levels in exhaled breath a fast, non-invasive diagnostic method for treatment of patients with liver and kidney disorders, is feasible. The NH 3 concentration measurements were performed with a 2f wavelength modulation quartz enhanced photoacoustic spectroscopy (QEPAS) technique, which is suitable for real time breath measurements, due to the fast gas exchange inside a compact QEPAS gas cell. A Hamamatsu air-cooled high heat load (HHL) packaged CW DFB-QCL is operated at 17.5°C, targeting the optimum interference free NH 3 absorption line at 967.35 cm -1 (λ∼10.34 μm), with ∼ 20 mW of optical power. The sensor architecture includes a reference cell, filled with a 2000 ppmv NH 3 :N 2 mixture at 130 Torr, which is used for absorption line-locking. A minimum detection limit (1σ) for the line locked NH 3 sensor is ∼ 6 ppbv (with a 1σ; 1 sec time resolution of the control electronics). This NH 3 sensor was installed in late 2010 and is being clinically tested at St. Luke's Hospital in Bethlehem, PA.