AIM: To evaluate whether the urinary creatinine concentration is a reliable reference value to standardise urinary solute excretion in a spot urine sample during the first week of life. METHODS: Spontaneously voided urine specimens were obtained in 48 healthy, full term neonates, aged 1 to 6 days (median 2.4) and in 168 healthy older children with a median age of 1.5 years (range 1 month to 3 years). In 62% of the children two urine samples were available with an interval of 2 to 4 (neonates) and 7 days (older children). RESULTS: In neonates both the urinary creatinine concentration and the urinary creatinine:osmolality ratios were significantly higher than in the older children, and were spread over a wider range. During the first postnatal week of life the mean urinary creatinine and urinary creatinine: osmolality ratio values in the first urine samples were also significantly higher than in the second samples. In children aged between 1 month and 3 years of age, these data were remarkably stable without any significant changes between repeat urine samples. CONCLUSIONS: The urinary creatinine concentration during the first days of life is high and variable, even when corrected for urinary osmolality. This is the opposite of what is found in older children and adults. Urinary creatinine and the urinary creatinine:osmolality ratio, therefore, cannot be used to standardise the urinary excretion of solutes in the first week of life.