An 8-year-old boy presented with a history of congenital hydronephrosis. The postnatal sonogram showed an enlarged right kidney with cortical thinning and markedly dilated calyces without associated pelviectasis (Fig. 1). Intravenous pyelogram confirmed the marked right calyceal dilatation. The pelvis and ureters were normal and there was no evidence of obstruction, with prompt and fairly symmetric opacification of the upper collecting systems (Fig. 2).

Fig. 1
figure 1

US image of the right kidney

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Fig. 2
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Intravenous pyelogram

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Congenital megacalyces is a rare cause of renal medullary dysplasia. The dilated calyces appear faceted and polygonal in contrast to the ballooned and spherical calyces of obstructive hydronephrosis. This condition is benign and nonprogressive, with near-normal renal function. The key to distinguishing this is that the pelvis and ureter appear normal without evidence of obstruction. It is important to differentiate this rare condition from the more common obstructive hydronephrosis to avoid unnecessary surgical intervention.