Determination of urinary estriol as a measure of placental function and intrauterine frtal well-being has been evaluated in obstetrics and has proved to be diagnostic value. To the aauthor`s knowledge, there has not been a report of urinary estriol excretion in pregnancy and some complicated pregnancies as well. Comparison with the results of various author`s is also done. It is also designed to evaluate clinical applicability and practicability. Material and Method Subjects were 241 pregnant women who were registred in prenatal clinic of the department of obstetrics of Yonsei Univeersity College of Medicine. 308 determinations were carried out from the 20th week to 45th week of gestation. Determination of urinary estriol excretion was carried out by the modified Brown`s method(1957) with a 24 hour`s urine specimen. A total of 80 complicated pregnancies were tested as follows; 1. 28 cases of toxemia of pregnancy(severe form of preeclampsia and eclampsia)and hypertension. 2. 6cases of pyelonephritis. 3. 6cases of hydatidiform mole. 4. 14cases of known intrauterine fetal death and 7cases of intrauterine fetal death that occurred during treatment of complications after admission. 5. 18cases of overdue. 6. 2cases of anencephalus. In twelvw of 161 normal pregnancies, urinary estriol value has been followed biweekly from 20th week`s gestation to term. Conclusion The results of urinary estriol excretion determination that are carried out are summarized as follows; 1. In normal pregnancies, there was a gradual increase in urinary estriol excretion and mean value of urinary estriol was 4.3mg/24hrs at 20th week gestation and 28.9mg/24hrs at term. 2. When pregnancy was complicated with hypertension, toxemia ro pyelonephritis, the urinary estriol excretion level was lower than normal pregnancy. 3. If the value of urinary estriol excretion at term is less than 4.0mg/24hours, intrauterine fetal death can be diagnosed and if more than 12mg/hour, fetal well-being can be diagnosed. 4. Urinary estriol excretion was markedly decreased in the hydatidiform mole and anencephalus. 5. Urinary estriol excretion was decreased in overdue pregnancy and the normal loweir limit of the urinary estriol excretion was 12.0mg/24hours. 6. Urinary estriol excretion of 12mg/24hours or more at term suggested mature fetus.