We have measured urinary excretion of free immunoreactive oestrone and oestradiol and their respective glucuronides in relation to creatinine in early morning samples in 132 fit, active postmenopausal women. None of the oestrogen/creatinine ratios was significantly correlated either with age or with years since menopause. However, we did demonstrate significant positive correlations between the levels of all four oestrogens and the body mass index, weight and fat mass. These results are similar to those obtained by other workers for plasma or serum oestrogen levels. Since assessemnt of oestrogenic status from plasma or serum may call for several samples to allow for the rapid minute-to-minute variation in levels, urinary oestrogen assays as described may provide a valuable non-invasive measurement of oestrogenic status. Such measurements may have a place in the identification of women at greater risk of developing symptomatic osteoporosis in later life.