Some recent studies have indicated that measurement of urinary GH (U-GH) excretion may be a useful tool for the evaluation of GH insufficiency in children with growth disorders, although some investigators are skeptical about the diagnostic value of U-GH. Most current assays are only available for specific laboratories or require time-intensive pretreatments of the specimens. This limits the possibility for many centers to compare their patients' data with others or to establish their own reference ranges for U-GH excretion. Therefore, we investigated the performance of a commercially available kit, which allows direct measurement of U-GH in untreated urine specimens. We established a reference range for the geometric mean of 3 morning urine samples in 446 healthy children and 71 adults. U-GH could be determined in all but 9 of 1526 samples (99.4%). U-GH excretion was significantly dependent on pubertal maturation (P <0.001) and sex (P <0.001), whereas age had no significant influence in the prepubertal group (P > 0.3). Peak values occurred in Tanner stages 3 and 4 (369 and 391 pg/h in females; 503 and 882 pg/h in males), corresponding to an age interval of 11-18 yr in boys and 9-15 yr in girls. Short collection periods (<6 h) were related to low values for U-GH excretion (nanograms per night; P <0.02). This time effect disappeared if U-GH excretion was expressed as picograms per h. If U-GH was related to creatinine output, there was a decrease in U-GH excretion during prepuberty, a blunting of the pubertal peak, and lower values in adults than in prepubertal children (P <0.0002). The intraindividual variation in U-GH excretion (picograms per h) ranged from 40-61%, constituting approximately two thirds of the interindividual variation. This variation was not lowered by relating U-GH to creatinine. We conclude that the assay was suitable for measurement of U-GH excretion in virtually all healthy volunteers. Sex and pubertal stage as well as urinary volume and clock times for collection periods should be registered when establishing a reference range for U-GH excretion and applying it for clinical purposes. Our reference values may be useful for further studies of patients with GH disorders.