Serial maternal urinary excretion of estriol and pregnanediol in 329 patients in their last trimester of pregnancy are presented and compared. Most pregnancies were clinically normal, although some had obstetric complications such as hypertension, preeclampsia, suspected intrauterine growth retardation, or gestation prolonged beyond 42 weeks. In 220 patients, both estriol and pregnanediol excretion were above the 10th percentile levels. The gestation at delivery varied between 32 weeks and 44 weeks. In this group, the incidence of fetal distress during labor was 5.5%, the incidence of fetal growth retardation was 5.9%, and the perinatal wastage was 0.9% (P<0.001). In 30 patients with normal estriol excretion, the pregnanediol excretion was below the 10th percentile, and there were no perinatal losses. Only 2 patients showed fetal distress in labor, and 2 infants were growth retarded. In 26 patients with normal pregnanediol but low estriol levels, 9 infants showed intrauterine fetal distress (35%) and 9 infants (35%) were growth retarded (P<0.001). There were 53 patients in whom both estriol and pregnanediol excretion was subnormal. Intrapartum fetal distress was present in 21 patients (40%), 13 infants showed evidence of intrauterine growth retardation (25%), and there were 4 perinatal deaths (7.6%) (P<0.001). In conclusion, the fetus is likely to be in jeopardy when both estriol and pregnanediol excretion are subnormal, and serial pregnanediol levels arc likely to be of assistance in assessing fetal well being when urinary estriol levels are low due to causes other than intrauterine growth retardation, such as treatment with steriods or ampicillin, or rarely, placental sulfatase deficiency.