An analyzer aimed for single-breath detection of exhaled endogenous NO is described. It is based on lead-salt MBE tunable diode lasers (TDL) operating at 5.4 micrometers spectral region. Specially designed high sensitive and fast acquisition system used in combination with the Herriott multipass gas cell allows 1 ppb concentration sensitivity to exhaled NO with about 0.5 s temporal resolution. Careful selection of the used analytical spectral region ensures high NO detection selectivity in the conduction if elevated water vapor and carbon dioxide content in exhaled air. Due to the TDL high spectral resolution the analyzer is also insensitive to the other numerous endogenous substances that could be present in exhaled air. The TDL NO analyzer was tested in measuring endogenous NO content in exhalation of patients suffering from different pulmonary diseases as well as of healthy control subjects. Some single-breath NO trace specific in asthma and COPD was observed allowing differentiation of these diseases with the exhalation NO test.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.