Clara cell protein (CC16) is a 16-kD protein secreted at the surface of respiratory airways by nonciliated bronchial and bronchiolar cells, including Clara cells. Using the same immunoassay as that recently developed for CC16 in lung lavage, we have measured CC16 in amniotic fluid samples from 100 normal fetuses and 51 fetuses with various pathologies. Ouchterlony immunodiffusion analysis showed a complete identity between CC16 in amniotic fluid and the protein in lung lavages of adults. CC16 was detectable in amniotic fluid from about the 15th wk of pregnancy, then progressively increased until delivery, with a tendency to reach a plateau after the 30th wk. Between the 15th and the 39th wk of pregnancy, the concentration of CC16 in amniotic fluid increased on average 25 times. The sex of the fetus did not influence the concentration of CC16 in amniotic fluid. Compared with expected values, levels of CC16 in amniotic fluid were on average not significantly altered in cases of spina bifida (n = 9), anencephaly (n = 7), and trisomy 21 (n = 6). In contrast, CC16 was on average significantly decreased in cases of diaphragmatic hernia (n = 6), trisomy 18 (n = 14), Turner syndrome (n = 4), and diabetic pregnancy (n = 5). In cases of diaphragmatic hernia, a relation emerged between the concentration of CC16 in amniotic fluid and both the weight of the lungs and the survivorship of the fetuses. The time course of CC16 in amniotic fluid during normal pregnancy and its reduction in pathologies associated with lung hypoplasia suggest that CC16 in amniotic fluid might serve as a marker of bronchial epithelium growth.