Abstract The natural history of primary megaureter was evaluated in 26 infants (20 boys, 6 girls) with 38 nonrefluxing primary megaureters. Prenatal ultrasound diagnosis had been made in 21 infants. In all patients, primary megaureter was associated with pelvicaliceal dilatation. Two patients with urinary tract infection and two with agenesis of the contralateral kidney underwent surgical treatment. Asymptomatic infants with good differential renal function on DTPA scanning were managed conservatively regardless of their postfurosemide drainage curve. One of these infants later developed a slight deterioration in function and was submitted to surgery. One patient was lost to follow-up. Twenty patients, after a period ranging from 4 months to 8 years, showed partial or complete regression of ureteral dilatation with stable good renal function. In conclusion, many asymptomatic primary megaureters in infancy improve spontaneously and do not require surgical treatment.