Urinary albumin excretion during pregnancy has been studied using a sensitive radioimmunoassay. One hundred pregnant women attending a high-risk antenatal clinic and 14 normal pregnant women were investigated serially, during pregnancy and post-partum. The normal subjects showed a small but significant rise in albumin excretion in the third trimester, which was sustained pre-delivery and in the first postnatal week. Twenty-six women were classified as having mild pre-eclampsia and 44 as having chronic hypertension without evidence of superimposed pre-eclampsia. In neither group was there evidence of proteinuria by conventional testing, nor was the median albumin excretion different from normal antenatally; in the first week after delivery a significant increase was observed, but this regressed to normal 6 weeks later. Eight patients developed severe pre-eclampsia, of whom one had evidence of underlying renal disease. Three presented with proteinuria already established. In the remaining five patients, the shift from normal to high albumin excretion occurred rapidly, usually preceded by a rise in uric acid and a decrease in the platelet count. These data suggest that proteinuric pre-eclampsia, as defined by relatively insensitive routine laboratory measurement, is not preceded by a phase of increasing albumin loss which can be detected by more sensitive assays.