The purpose of this study was to determine if a relationship existed between amniotic fluid volume and fetal urine output in 134 uncomplicated pregnancies at 38-43 weeks of gestation. The semiquantitative amniotic fluid index was used as a clinical estimate of amniotic fluid volume, and fetal urine output was determined by measuring the fetal bladder volume every 3-5 min for approximately 20-30 min. No correlation was found between the hourly fetal urine production rate and the amniotic fluid index (P = 0.34)- However, a statistically significant (P = 0.006) relationship was found between a urine production rate ≤30 ml/hr, which was approximately the lower tenth percentile in this study population, and a diagnosis of oligohydramnios. Nevertheless, of the 51 patients with a fluid index ≤8 cm, only 12 (24% had an hourly fetal urine production rate at or below 30 ml/hr. Conclusions: (1) There appears to be no significant relationship, in general, between single measurements of the amniotic fluid index and the rate of fetal urine production at 38-43 weeks of gestation; and (2) oligohydramnios is only occasionally associated with an hourly urine production rate below the tenth percentile, suggesting an etiology for the reduced fluid volume other than a significant decrease in fetal urine output in the majority of patients with oligohydramnios. © 1993 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.