The tremendous interpatient variations that occur for oestrogen excretion during pregnancy are largely overcome by expressing results in terms of relative concentrations (i.e., as percent of total oestrogens excreted). Specimens of urine from 28 pregnant patients with a variety of complications were assayed for their oestrogen content by an "oestrogen profile" technique and the results thus expressed were compared with those from 20 normal pregnancies. Statistically significant changes are shown in the group of patients in the "above" normal range (normal range = normal mean +/- 3 X SE) for five oestrogens (oestrone, 2-hydroxyoestrone, 2-methoxyoestrone, 2-hydroxyoestradiol, and 2-hydroxyoestriol) and "below" the normal range for one oestrogen (oestriol). When the means of the oestrogen values for the different groups of patients were examined, two things became obvious; firstly, only patients with hypertension showed these changes, and secondly, two additional oestrogens (16-hydroxyoestrone and 16-epioestriol) were excreted in large amounts by the normotensive "high risk" patients. A comparison of the two methods of expressing oestrogen assay results is made quantitatively (as milligrams per day) versus relatively (as percent of total oestrogens excreted), and advantages and disadvantages of each are discussed.