BACKGROUND AND OBJECTIVES: Proteinuria is associated with chronic kidney disease (CKD), and heavy proteinuria predicts a rapid decline in kidney function. However, the epidemiologic distribution of this important biomarker study is not well described in the pediatric CKD population. Design, setting, participants & measurements: This cross-sectional study of North American children with CKD examined the association of proteinuria among the baseline clinical variables in the cohort. Urinary protein-to-creatinine ratios (Up/c) were used to measure level of proteinuria. RESULTS: Of the 419 subjects studied, the median GFR as measured by iohexol disappearance (iGFR) was 42 ml/min per 1.73 m(2), median duration of CKD was six yr, and glomerular diseases accounted for 22% of the CKD diagnoses. Twenty-four percent of children had normal range (Up/c <0.2), 62% had significant, and 14% had nephrotic-range proteinuria (Up/c >2.0). A decrease in iGFR was associated with an increase in Up/c. At any level of GFR, a higher Up/c was associated with a glomerular cause of CKD and non-Caucasian race. Among subjects with a glomerular cause of CKD, Up/c was lower in subjects reporting utilization of renin-angiotensin system (RAS) antagonists (median Up/c = 0.93) compared with those who did not (median Up/c = 3.78). CONCLUSIONS: Proteinuria is associated with level of iGFR, cause of CKD, and race. The longitudinal study design of Chronic Kidney Disease in Children (CKiD) cohort study and the large number of subjects being studied has created an opportunity to better define the association between proteinuria and CKD progression.