Annexin V has a molecular weight of 32-35 kDa and has been reported to possess anticoagulant activity, inhibition of phospholipase A(2), regulation of membrane transport, proliferation and signal transduction. It is reported that urinary annexin V concentration may be an indicator of apoptosis and acute renal injury related to the urinary protein level. The aim of this study was to define the role of urinary annexin V excretion and serum annexin V concentrations as new prognostic tools and follow-up criteria in children with steroid-sensitive (SSNS) and steroid-resistant nephrotic syndrome (SRNS). Annexin V concentrations were measured in serum and 24-h urine samples in 23 SSNS patients in both relapse and remission periods of each patient and in 22 SRNS patients and 22 healthy controls. Total protein, albumin, blood urea nitrogen (BUN), creatinine, total cholesterol concentrations, and 24-h urinary excretion of protein and creatinine were also measured in each patient. In the SRNS group, median 24-h urinary annexin V levels were significantly higher than for all other groups (5,048.8 ng/g creatinine vs. 2,839.5 ng/g creatinine in SSNS relapse group; 2,500.0 ng/g creatinine in SSNS remission group, and 2,018.3 ng/g creatinine in healthy control group). No significant correlation was found between urinary protein excretion and 24-h urinary annexin V levels in all subjects. Twenty-four-hour urinary annexin V excretion may be a predictor in children with SRNS, and it may be a prognostic marker in children with NS.