The immaturity of the fetal lung and the resulting respiratory distress syndrome are the main problems of obstetrics. Over a long period of time the only possibility to recognize the maturity of fetal lungs certainly was the determination of the L/S-ratio by amniocentesis. Two years ago first results were presented to determine fetal lung maturity by sonography. We examined 428 patients between 29 and 40 completed weeks of gestation. By comparing the characteristics of frequency of fetal lung echoes to those from the fetal liver as a reference organ it becomes possible to standardize fetal lung changes with gestation. In one ultrasound section we depicted lung and liver as well. According the A-mode we registered frequencies of both. The registered frequencies were entered digitally into a computer to evaluate the mean, maximal and minimal frequencies. Afterwards quotients of the frequencies of fetal lung and liver were calculated. For all weeks of gestation, mean values and standard deviations were made. It has been shown, that the liver is an adequate reference organ since the reflection pattern between the different weeks of gestation does not change, however significant changes have been seen in the fetal lungs. Q-mean, the quotient of fetal lung and liver reflection can be calculated. A quotient of Q-mean < 1.10 suggests lung maturity, while values > 1.10 speak for immaturity. Ultrasound examinations with 55 expectant women were compared with the lung conditions after delivery. 30 examinations were correlated with the results of the L/S-Ratio by amniocentesis.(ABSTRACT TRUNCATED AT 250 WORDS)