To date only 2 cases of urinary retention secondary to a müllerian duct cyst have been reported in infancy. We report a third case of bladder outlet obstruction caused by massive dilatation of persistent müllerian remnant in a 2 month old boy, the youngest to date. Surgical intervention was done because patient continued to have retention following conservative management. The cyst was large, located in the midline, and was associated with an absent left kidney.