One thousand six hundred and sixty pregnancies were recorded in this investigation. In 1042 women with singleton pregnancies and known term, measurements of the total serum estriol (E3) were made exclusively for the present study. The study was prospective, and the clinicians had no knowledge of the E3 values. Within the screened group a low E3 value entailed a 42% risk of delivering an infant with perinatal complications and a 27% risk of an infant small for gestational age (SGA). 7% of the infants with perinatal complications and 19% of the SGA infants were detected at the screening. In a sub-group of 800 pregnancies considered normal according to given criteria, a low E3 level involved a 27% risk of perinatal complications and a 15% risk of an SGA infant. Of these cases 4% and 14% respectively had been detected at the screening. It is concluded that the E3 level should be measured on wide indications, but that this test is of little value in pregnancies that are clinically quite normal. Thus, the benefit of screening all apparently normal pregnancies depends upon the extent and thoroughness of the clinical antenatal care.