Twin-twin transfusion syndrome (TTTS) complicates one in five monochorionic pregnancies and is generally associated with high mortality and morbidity. One twin (the recipient) grows appropriately and has polyhydramnios while the other (the donor) may have a reduced growth velocity and severe oligohydramnios. The disparities in amniotic fluid volumes represent differences in fetal urine output. These differences occur secondary to hemodynamic changes, in which the vascular arrangement of placental anastomoses in TTTS leads to unidirectional flow from the donor to the recipient twin. A better understanding of the pathophysiology may contribute to improved management of this morbid condition. We studied three consecutive prospectively diagnosed stillborn twin pairs affected by early-onset TTTS. Renin gene expression was studied in sections of fetal kidneys with immunocytochemistry using a renin antiserum and with in situ hybridization using riboprobes complementary to renin mRNA, and renin-secreting cells (RCC) were counted. The overall maturation of the renal cortex was assessed by the percentage of immature glomeruli. The donor twin kidneys were smaller than those of the recipients, but the maturation of the renal cortex was not significantly different (28.2% immature glomeruli in the donor and 24.4% in the recipient kidney). The donor kidney showed increased renin gene expression with hyperplastic juxtaglomerular apparatuses (JGAs) that contained excess RCCs (median 20.02 [25th-75th centiles, 5.4, 25.1 RCCs per 100 glomeruli]). In contrast, the recipient kidney was virtually devoid of these cells (0.04 [0, 0.36] RCCs per 100 glomeruli; P < 0.05). In the donor kidney, increased renin release may, by a local action, contribute to renal vasoconstriction and oliguria. Increased renin and/or angiotensin II in the blood passing through the placental anastomoses may, by an endocrine action, suppress renin synthesis in the recipient kidney, thereby increasing renal blood flow and causing polyuria and polyhydramnios. These changes in the renal RAS could thus contribute to the pathogenesis of TTTS. The renal renin changes noted here may represent a contributory or compensating mechanism, the success of which may dictate the overall survival of the twin pregnancy and allow better understanding of the pathophysiology and perhaps therapy that may be employed in this condition.