Soy intake early in life may protect against breast cancer later in life, possibly by altering sex hormone metabolism. We evaluated the feasibility of assessing urinary sex steroid excretion among 20 young girls aged 8-14 yr in an 8-wk trial. The girls consumed one daily soy serving, collected weekly overnight urine samples, and reported Tanner stages for breast and pubic hair development. Sex steroid excretion was measured in duplicate by gas chromatography-mass spectrometry and adjusted for urinary creatinine. The respective coefficients of variation for estrone, estradiol, estriol, testosterone, pregnanediol were 11.4%, 10.4%, 8.4%, 12.8%, and 4.6%. The statistical analysis included t-tests, Spearman's correlations, and analysis of variance. Seventeen girls completed the study and showed good compliance with the intervention strategy. We observed nonsignificant increases in total androgens (0.11 microg/mg creatinine) and total estrogens (0.001 microg/mg creatinine) and a nonsignificant decrease in pregnanediol (-0.03 microg/mg creatinine) during the study period. Higher Tanner stages for pubic hair development were associated with ninefold higher estrogen, fourfold higher androgen, and twofold higher pregnanediol excretions (P=0.01, P<0.001, and P=0.047, respectively). Similar differences were observed after stratification by breast development and menarcheal status. The association of sex steroid levels with pubertal development supports the validity of the sex steroid measurements.