In the absence of specific symptomatology in children, the early diagnosis of acute pyelonephritis is a challenge, particularly during infancy. In an attempt to differentiate acute pyelonephritis from lower urinary tract infection (UTI), we measured serum procalcitonin (PCT) levels and compared these with other commonly used inflammatory markers. We evaluated the ability of serum PCT levels to predict renal involvement, as assessed by dimercaptosuccinic acid (DMSA) scintigraphy. Serum C-reactive protein (CRP), leukocyte counts, and PCT levels were measured in 64 children admitted for suspected UTI. Renal parenchymal involvement was assessed by (99m)Tc-DMSA scintigraphy in the first 7 days after admission. In acute pyelonephritis, the median PCT level was significantly higher than in the lower UTI group (3.41, range 0.36-12.4 microg/l vs. 0.13, range 0.02-2.15 microg/l, P<0.0001). In these two groups, respectively, median CRP levels were 120 (range 62-249 mg/l) and 74.5 (range 14.5-235 mg/l, P=0.012) and leukocyte counts were 15,910/mm(3) (range 10,200-26,900) and 14,600/mm(3) (range 8,190-26,470, P=0.34). For the prediction of acute pyelonephritis, the sensitivity and specificity of PCT were 94.1% and 89.7%, respectively; CRP had a sensitivity of 100%, but a specificity of 18.5%. We conclude that serum PCT may be an accurate marker for early diagnosis of acute pyelonephritis.