In 901 pregnancies between 1978-1981 a total of 5281 serum estriol determinations were performed. 596 gravidas were high-risk patients (diabetes mell., toxemia, IUGR, premature labour, prolonged pregnancy, Rh incompatibility). In 142 of these high-risk pregnancies the estriol values were below the 10th percentile or the estriol decrease extended 40%. These cases were analysed individually, in order to answer the question, how often serum estriol values were decisive for active obstetrical management. Our data show that a drop of estriol over 40% was very rare (0.53%). For the newborn such an estriol-drop does not mean higher perinatal morbidity or mortality in any case. The endocrine parameters were not very often decisive for an active obstetrical intervention. Probably because of the large fluctuations and day-to-day-variations of the estriol values mainly due to drugs which were widely used in risk pregnancies. These exogenous factors had a negative effect on the clinical value of the serum estriol determinations.