TCF2, the gene encoding for hepatocyte nuclear factor 1beta, is involved in early renal development. Mutations in TCF2 lead to heterogeneous renal phenotypes. Antenatal ultrasonography may show unilateral/bilateral hyperechogenic or enlarged cystic kidneys. In children or adults, cystic renal hypoplasia/dysplasia is a common feature, occasionally associated with maturity-onset diabetes of the young type 5 and genital tract abnormalities. We report an unusual presentation characterized by massively enlarged polycystic kidneys mimicking autosomal dominant polycystic kidney disease in monozygotic twins. Bilateral enlarged cystic kidneys were discovered in week 13 of a gemellic pregnancy. Postnatally, kidney size increased in both children, reaching 16 cm at 20 years. Nephromegaly was associated with bilateral cysts and a slowly decreasing glomerular filtration rate (40 mL/min/1.73 m(2) at 20 years). There was neither pancreatic nor genital malformation. Non-type 1 diabetes mellitus was diagnosed incidentally in both twins at 20 years. Knowledge of early-onset diabetes (at age 19 years) in their father prompted us to search for the TCF2 mutation. Genetic analysis showed complete TCF2 heterozygous whole-gene deletion in both twins. Genetic testing could not be performed in the father. Bilateral massively enlarged polycystic kidneys mimicking autosomal dominant polycystic kidney disease in young adults may be related to TCF2 mutation. Although uncommon, this new phenotype enlarges the clinical spectrum of kidney involvement associated with TCF2 mutation. In this case, maturity-onset diabetes of the young-type diabetes paved the way to accurate diagnosis.