We describe one case of long term post-natal follow-up of hyperechoic fetal kidneys related to HNF-1β mutation with cystic changes over a 9-year period in a female patient. This diagnosis was suspected on the basis of the renal US findings and was confirmed by complementary genetic examination. After birth, cortical cysts were detected and at the age of 4, medullary cysts were found, that disappeared with time. Currently our patient displays hyperechoic kidneys with only cortical cysts. This case report highlights the variability of US appearances in relation with HNF-1β genetic mutation.