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). In pleural effusions, the CC16 concentration appears to be associated with its diffusion from the lung as evidenced by high CC16 levels in cardiac pleural congestion.