Two cases of prenatally identified urinoma associated with an isolated hydronephrosis are presented, and the pathophysiology and prognosis of this rare condition are discussed. The presence in utero of a peri-renal collection associated with an isolated hydronephrosis seems to be a sign of significant renal dysplasia. These urinomas disappear spontaneously, thus drainage is not necessary, except in the case of compression of surrounding structures. The functional prognosis of these kidneys seems to be most unfavourable.