Human Clara Cell Protein belongs to the family of secretoglobins and is a secreted protein product of nonciliated bronchiolar Clara cells. There is convincing data suggesting that it has phospholipase A2 inhibitory activity, as well as, a number of other immunomodulatory features including inhibition of interferon gamma signalling and Th1 vs. Th2 lymphocyte regulation. It was proposed as a potential peripheral marker of respiratory epithelial injury and bronchial dysfunction. Clara Cell Protein 16 concentrations have been determined in both serum and bronchoalveolar lavage fluid in numerous studies since 1994. In serum, its increase is associated with age, asbestos, nitrogen chloride and ozone exposure, sarcoidosis and high PEEP ventilation. Decreased serum CC16 levels are found after pulmonary resection, in silica-exposed workers, smokers and in asthma. Decreased CC16 concentrations were also found in the amniotic fluid of fetuses suffering from pulmonary hypoplasia caused by various mechanisms (diaphragmatic hernia, diabetic fetopathy, Turner and Down syndrome).