To study the feasibility of noninvasive sampling in pediatric patients, we examined the concentrations of LH and FSH in paired serum and urine samples from 65 children (age 0–15 y) with highly sensitive time-resolved immunofluorometric assays. The detection limits of the assays were 0.015 lU/L for LH and 0.018 lU/L for FSH. These sensitivity levels allowed quantification of the low prepubertal LH and FSH concentrations. The correlation between serum and urine gonadotropin values was very good (r = 0.751, p < 0.001 for FSH; and r = 0.720, p < 0.001 for LH), and the urine and serum concentrations were very similar. Correction of urinary gonadotropin concentrations for changes in urinary flow by standard methods using density [concentration ± (0.02/density - 1)] or creatinine (concentration/creatinine) did not improve the correlation. Therefore, measurement of urinary gonadotropins without correction can simply be used in the pediatric outpatient setting as a noninvasive alternative to serum determinations.