Unilateral renal agenesis is a relatively common congenital urinary malformation that is usually diagnosed during fetal ultrasonography. Some cases of URA may represent involution of a previous MCDK. Unilateral renal agenesis may be an isolated congenital malformation or may be associated with chromosomal abnormalities or a variety of nonchromosomal syndromes including the VACTERL and MURCS associations. Congenital cardiac malformations are the most common malformations associated with URA. Girls with URA should have a pelvic ultrasound to look for abnormalities in the müllerian structures. Vesicoureteral reflux is the most common abnormality noted in the contralateral kidney. The prognosis of individuals with isolated URA is good. There are reports of hypertension and renal insufficiency in long-term studies of patients with a single kidney, and lifetime follow-up is recommended.