A patient with group C polycystic kidney disease had abnormally high concentrations of total serum estriol (E3) but low-normal urinary levels of E3 throughout the period of study (20 weeks of gestation until delivery by cesarean section at 33 weeks, 5 days). At delivery and at regular intervals until 6 hours thereafter serum specimens were analyzed for unconjugated E3 and its four major conjugates. Comparisons were made with levels in three normal volunteer subjects studied in the same way. In the 6 hours, total E3 declined 37% in the subject with polycystic kidney disease whereas in normal subjects the decline ranged from 84% to 99%. Unconjugated E3 was depleted from the serum in all subjects in about 2 hours. The major difference between the patient with polycystic kidney disease and the normal subjects was in the profile of E3 conjugates. In polycystic kidney disease, E3-3-glucosiduronate (E3-3G) and E3-3-sulfate-16-glucosiduronate (E3-SG) respectively made up 83% and 1.8% of the total serum estriol, whereas in the normal subjects the average values were 13% for E3-3G and 49% for E3-SG. There were no consistent dramatic changes in the percentage contribution of any conjugate to the total E3 level in either the patient with polycystic kidney disease or the normal subjects in the predelivery or postdelivery periods. The E3 profile in polycystic kidney disease is explainable in terms of impaired renal function coupled with normal enterohepatic metabolism of E3.