The lecithin/sphingomyelin-ratio (L/S-ratio) in amniotic fluid has become established in recent years as a valuable laboratory parameter for determining foetal lung maturity and has thus helped to avoid the neonatal respiratory distress syndrome. Since glucocorticosteroids are a decisive factor in stimulating foetal lung development, we were prompted to investigate whether the determination of cortisol in amniotic fluid or maternal blood could also serve as an indicator of foetal lung maturity. The cortisol concentration was measured by radioimmunoassay in amniotic fluid (n = 91) and in maternal venous plasma (n = 70) from normal and complicated pregnancies and was compared with the L/S-ratio. A significant correlation between gestational age and cortisol in amniotic fluid (r = 0.711) and in maternal venous plasma (r = 0.625) was observed. There was also a significant correlation between the L/S-ratio and amniotic fluid cortisol (r = 0.701). However, compared to the L/S-ratio, measurements of cortisol yielded 10% false positive and 11% false negative results. A significant correlation between the L/S-ratio and the maternal plasma cortisol could also be demonstrated (r = 0.631). Here the comparison with the L/S-ratio gave even poorer results: 16% false positive and 6% false negative. The high percentage of false results, especially false positive results yielded by cortisol estimations in both fluids leads to the conclusion that the L/S-ratio remains the superior diagnostic parameter in determining foetal lung maturity.