This paper reports on the association between renal sinus hyperechogenicity and acute pyelonephritis. The medical records and imaging studies of 18 children displaying this pattern were retrospectively studied. Thickening of the renal pelvis and renal enlargement were the most frequent associated sonographic abnormalities. Further subtle findings can be found on sonography and colour/power Doppler. Their identification can help in the diagnostic approach to acute pyelonephritis and may obviate the need for other imaging modalities such as enhanced CT or 99mTc-DMSA scintigraphy. Renal sinus hyperechogenicity was also identified in a parallel study performed in female rabbits with experimental pyelonephritis and was shown, histologically, to be related to exudates of fibrin and polymorphonuclear leukocytes, interstitial oedema and micro-abscesses.