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Purpose: We determined normal, age related reference data regarding maximum voided volume and nocturnal urine production using the same methodology as in clinical practice. Materials and Methods: A total of 62 girls and 86 boys without enuresis (mean ± SD age 9.64 ± 2.63 years, range 3 to 15) completed 4 days (2 weekends) of frequency-volume charts and 14 days of home recording of nocturnal urine production. From these recordings maximum voided volume with and without first morning void was derived for each subject. Also, average nocturnal urine volume with and without nocturia was calculated. Percentiles were produced by dividing the population into 1-year age groups. Results: Based on 2,836 daytime voids and 1,977 overnight recordings, maximum voided volume and nocturnal urine volume showed a significant linear relationship with age but not with gender. Maximum voided volume with first morning void was significantly higher than without (403 ± 137 ml vs 281 ± 112 ml, p <0.0001) and the 50th percentile line of maximum voided volume with first morning void was 80 to 100 ml higher than Koff''s formula (30 × [age + 1] ml). Conversely the 50th percentile of maximum voided volume without first morning void was almost identical to Koff''s formula. Regarding nocturnal measurements, nocturia was noted on 128 nights (6.5%) and nocturnal urine volume on nights with nocturia was significantly higher than on nights without nocturia (365 ± 160 ml vs 248 ± 75 ml, respectively, p <0.0001). The 97.5th nocturnal urine volume percentile line of healthy children deviated markedly from the current International Children''s Continence Society definition of nocturnal polyuria, especially at low and high ages. Conclusions: We demonstrate clearly that the universally used formula 30 × (age + 1) ml is indeed valid for a population of healthy Danish children but only if the first morning void is disregarded. Furthermore, we question the validity of the current International Children''s Continence Society formula for nocturnal polyuria (nocturnal urine volume greater than 130% of maximum voided volume for age), and instead we propose the formula, nocturnal urine volume greater than 20 × (age + 9) ml. [Copyright &y& Elsevier]