Estriol concentration values in the first urine of 161 asymptomatic premature and mature newborns (mean birth weight, BW=3,312 g, range 1,350–4,320 g; mean gestational age, GA = 39.08 weeks, range 28–42 weeks) showed a positive correlation with increasing BW(r = 0.401, p < 0.001) as well as with GA (r = 0.432 p < 0.001). When urinary estriol levels were related to creatinine concentrations (estriol-creatinine ratio = E/C ratio) in order to correct for possible differences in renal functional capacity, the respective correlation coefficients were r = 0.307 (p < 0.001) for BW and r = 0.284 (p < 0.001) for GA. In a small group of unequivocal cases of the idiopathic respiratory distress syndrome (RDS) comprising a total of 13 infants (mean BW = 1,871 g, range 1,260–2,890 g, mean GA = 32.38 weeks, range 28–38 weeks) a statistically significant correlation could not be proved between urinary estriol levels and either, BW or GA (r = 0.487 for BW; r = 0.441 for GA). The reason for this may be the small number of cases tested. When urinary estriol levels are again related to urinary creatinine concentration, the correlation does become significant: r = 0.704, p < 0.01 for E/C ratio over BW; r = 0.738 p < 0.01 for E/C ratio over GA. For a given stage of maturity, estriol excretion (E/C ratio) is reduced in very premature RDS babies when compared with healthy controls. As maturity is approached, the differences gradually disappear. It is concluded that in the present series the pattern of increase in estriol excretion with advancing pregnancy is similar in healthy and in RDS patients. Thus a significant asphyxial insult contributing to the pathogenesis of the disease may have occurred during late labour and/or delivery. Had asphyxia been present prior to the onset of labour, a more pronounced drop in fetal urinary estriol and analogously in neonatal urinary estriol excretion would have had to be expected.