Relationship of vesicoureteral reflux (VUR), urinary tract infection (UTI) and subsequent renal scarring nowadays is still in question. This study aimed to determine 1) the incidence of cortical scarring in Thai children presenting with upper urinary tract infection, 2) the association between VUR with acute pyelonephritis and subsequent renal scarring, 3) the use of DMSA and direct radionuclide cystography (DRNC) scintigraphy in children with UTI. Sixty newly diagnosed UTI children underwent DRNC and Tc-99m DMSA renal cortical scintigraphy at the time of first UTI diagnosis and 6 months later. There were no significant differences of age, sex, type of antiobiotics and pathogens (E. coli vs non-E. coli) between those who did and did not develop scars (p>0.05). 58/98 of 1st DMSA abnormal kidneys (59.18%) developed scars. 60.20 per cent of 98 positive 1st DMSA had VUR while 80.33 per cent of 61 with VUR had positive 1st DMSA. 59.02 per cent of these 61 kidneys developed scars. Scar occurrence were 18.7 times in high grade VUR compared to low grades. In conclusion, there was a high incidence of acute pyelonephritis in the presence of VUR but acute pyelonephritis does not necessarily need VUR for its development. High grade reflux with upper UTI, is a strong indicator for renal scarring. Children presenting with UTI, irrespective of age, sex, or pathogen, should have both DMSA and DRNC scintigraphy performed to identify upper UTI and high risk patients who will develop subsequent renal scarring.