Osr1 is a transcriptional repressor that plays critical roles in maintaining the mesenchymal stem cell population within the developing kidney. Here, we report that newborn pups with a heterozygous null mutation in Osr1 exhibit a 21% incidence of vesico-ureteric reflux and have hydronephrosis and urinary tract duplications. Newborn pups have a short intravesical ureter resulting in a less competent uretero-vesical junction which arises from a delay in urinary tract development. We describe a new domain of Osr1 expression in the ureteral mesenchyme and within the developing bladder in the mouse. OSR1 was sequenced in one hundred and eighty-six children with primary vesico-ureteric reflux and seventeen have single nucleotide polymorphisms. Fifteen children have a common synonymous variant, rs12329305, one child has a rare non-synonymous variant, rs3440471, and one child has a rare 5'UTR variant, rs45535040. The impact of these SNPs is not clear, therefore, the role of OSR1 in human disease remains to be elucidated. Osr1 is a candidate gene implicated in the pathogenesis of vesico-ureteric reflux and CAKUT in mice.