Glomerular filtration rate (GFR) was estimated by serum creatinine (Schwartz's equation) and serum cystatin C (Filler's equation) in preterm neonates (24-31 weeks of gestation) in a prospective cohort study. Serum creatinine and cystatin C was obtained at birth and then every two weeks during the first month. We found a poor fit between two methods, and a steadier GFR assessment by cystatin C.