Total and unconjugated plasma estriol levels from the 24th to the 40th gestational week in 161 normal and 44 pathological pregnancies were studied. A lack of correlation between the two fractions was observed at the different gestational ages, probably due to the different rates of the several metabolic steps which condition the levels of the hormone fractions also in quite physiological conditions. The onset of pregnancy complications further modifies the ratio between the two fractions, as they differently affect the single metabolic processes which are essential for estriol to be produced, conjugated, recovered and excreted. E.P.H. gestosis and poor intrauterine fetal growth can be better diagnosed by the assay of total than unconjugated estriol: the total fraction proved to have the higher sensitivity, predictive value and relative risk and should therefore be preferred for pregnancy monitoring, as it better corresponds to the clinical situation.