OBJECTIVE To investigate the effect of demographics including age and sex on excretion of 4 key urinary factors (calcium [Ca], magnesium [Mg], oxalate and uric acid [UA]) related to kidney stone risk. METHODS Twenty-four-hour urine samples were collected from non-Hispanic white sibships in Rochester, MN. Height, weight, blood pressure, serum creatinine, and cystatin C were measured. Diet was assessed using the Viocare food frequency questionnaire. Effects of demographics and dietary elements on urinary excretions were evaluated in univariate, multivariate, and interaction models that included age, sex, and body mass index (BMI). RESULTS Samples were available from 709 individuals. In multivariate models, sex was a significant predictor of all 4 urinary factors, age was significant for all but UA excretion, and serum creatinine was significant only for Ca and Mg excretion (P<.05). BMI or weight positively correlated with Mg, oxalate, and UA excretion (P<.05). Use of a thiazide diuretic (lower) and dietary protein (higher) were associated with Ca excretion, whereas dietary Ca was associated with higher Mg excretion. Urinary UA excretion increased with animal protein intake and cystatin C estimated glomerular filtration rate (eGFR) and was lower with concurrent loop diuretic use. Significant interaction effects on urinary UA excretion were observed for loop diuretic use and sex, eGFR and sex, age and animal protein intake, and BMI and eGFR (P<.05). CONCLUSION Age and sex influence excretion of key urinary factors related to kidney stone risk and should be taken into account when evaluating kidney stone patients. (C) 2015 Elsevier Inc.