Congenital unilateral renal agenesis usually is an nonsyndromic, sporadic abnormality that is detected during prenatal ultrasonography. Unilateral renal agenesis is an important clinical finding combining multiple congenital anomalies in case that renal agenesis itself is a part of a syndrome or a hereditary renal dysplasia or in abnormal residual solitary kidney. The incidence is between 1/500 and 1/800 live births. Unlike bilateral renal agenesis, unilateral renal agenesis shows appropriate amniotic fluid, normally functioning kidney and visualization of the bladder by ultrasonography. Color Doppler ultrasonography can be used as an adjunct to the diagnosis of unilateral renal agenesis. We present two cases of unilateral renal agenesis diagnosed prenatally by color Doppler and ultrasonography with a literature review.