The urinary excretion of oestriol during normal pregnancy has been determined with Ittrich's direct method (i.e. without extraction). The mean excretion curve and the limits of the normal variations are presented. The diagnostic significance of oestriol estimation is compared with other diagnostic methods in various common obstetrical problems. The comparison between the urinary oestriol concentration and amnioscopy findings in 9 cases of postmaturity shows that the oestriol estimation gives a better forecast of subsequent events. Levels of oestriol excretion below 2 mg/d in the last trimester of pregnancy indicated foetal death in utero in 9 cases in which the radiological signs were absent. These were present in 2 other cases in which the normal levels of oestriol concentration indicated an undisturbed pregnancy. In 10 suspected cases of missed abortion the values of oestriol excretion proved to be in agreement with the results of the immunologic Pregnosticon and Ortho pregnancy tests. In 6 other cases in which there was disagreement the oestriol values reflected the placental activity more accurately. The cause of the disagreement has been investigated. The practical advantages of Ittrich's method are evaluated.