Objectives: To determine whether a circadian variation of urinary excretion of calcium and phosphorus exists in preterm infants. Study design: We studied 70 newborn infants (median birth weight 1920 gm, range 660 to 3550 gm; median gestational age 34 weeks, range 25 to 42 weeks) at a median postmenstrual age of 36 weeks (range 32 to 42 weeks). Within a period of 24 hours, four urine specimens were collected during 6-hour periods. The concentrations of calcium, phosphorus, sodium, potassium, and creatinine were determined and creatinine quotients were calculated for each specimen. Results: No clinically relevant circadian variation in urinary excretion for any of these minerals was found. Conclusion: If spot urine specimens are used to monitor calcium and phosphorus balance in preterm infants, the time of the day these are collected is not important.