The urinary excretion of oestriol in women with normal pregnancies has been estimated by means of two methods developed by Frandsen (1963). Method I has a sensitivity of 0.5 mg/24 hr of oestriol and method II a sensitivity of 25-50 [mu]g/24 hr. The excretion in 106 cases of threatened abortion was also investigated. It was found that abortion was almost inevitable if the excretion of oestriol was low, while 75% of the cases with a normal output continued to term. Often a discrepancy between the result of a gonadotrophin test and of the oestriol estimation was found. In such cases the oestriol estimation gave the most reliable guide to the condition of the pregnancy.