Plasma sulfate concentration (SO4) was determined in 38 non-dialyzed children with chronic kidney disease and compared to inulin clearance and to other parameters of renal function. SO4 was measured by atomic absorption spectrophotometry. Plasma SO4 was inversely correlated with GFR and effective renal plasma flow. Positive correlations were found between plasma SO4, BUN, serum creatinine and serum phosphate levels. The highest levels of plasma SO4 were observed in 15 children undergoing regular dialysis. It is suggested that sulfate retention might be involved in the pathogenesis of uremic acidosis and bone disease.