This study was designed to improve the definition of acute renal failure (ARF) in very preterm infants. Twenty-eight newborn infants with gestational age < or =32 weeks were prospectively studied in the first 5 days of life and made up a control group as they did not present risk factors for vasomotor renal insufficiency. Renal insufficiency was defined as an increase in daily serum creatinine concentration above the 99th interval limit obtained in this control group, i.e., 43 micromol/l on day 1 and/or 21 micromol/l on day 2 and/or 14 micromol/l/day on day 3 and/or 22 micromol/l/day on day 4. According to this definition, 20 very preterm infants with ARF were identified. As compared with the control group, the ARF group showed more prolonged oliguric episodes, lower diuresis, insufficient weight loss (in spite of a reduction in water intake) and also more episodes with natremia <130 mEq/l (35 vs. 0%; p <0.05) and/or kalemia >6 mEq/l (40 vs. 11%; p <0.05). Therefore, assessment of daily changes in serum creatinine concentration in very preterm infants allows the diagnosis of clinically significant reduction in glomerular filtration rate.