BACKGROUND Calcium-oxalate (CaOx) deposition and systemic oxalosis are uncommon in children with chronic renal failure (CRI), but frequent in children with primary hyperoxaluria type I (PH-1). We hypothesized a difference in plasma CaOx saturation (betaCaOx) and its determining factors would explain this discrepancy. METHODS Therefore, in addition to common biochemical measurements, plasma-oxalate (POx), citrate (PCit) and sulfate (PSulf) (plasma anions) were measured and betaCaOx was calculated in 17 PH-1 patients with normal renal function receiving pyridoxine and citrate therapy, in 54 children with CRI (SCr 0.9 to 5.9 mg/dl), and in 50 healthy children (NL). Plasma anions were analyzed by ion-chromatography and betaCaOx was calculated using a PC-based program for solution equilibria. RESULTS Compared to NL, all plasma anion levels and betaCaOx were higher in PH-1 and CRI; POx, PCit and betaCaOx were higher in PH-1 than in CRI (P < 0.05), but PSulf was higher in CRI (P < 0.01). BetaCaOx and POx were correlated in all groups (r = 0.63 to 0.95, P < 10(-4)). POx and betaCaOx were both inversely correlated to a decrease in GFR in CRI patients. PCit and PSulf did not influence betaCaOx. Although supersaturation (betaCaOx > 1) was found in 7 CRI and in 4 PH-1 patients, eye examinations were suspicious for CaOx depositions only in the PH-1 patients, while systemic oxalosis was confirmed in one PH patient because of oxalate osteopathy. CONCLUSIONS In PH-1, POx and betaCaOx are elevated even with normal renal function, which increases the likelihood of CaOx crystal deposition. Therefore, more effective therapy to decrease betaCaOx is crucial to reduce the risk of systemic oxalosis. In children with CRI unknown, but presumably protective substances, help prevent the risk of systemic oxalosis, despite increased POx and betaCaOx levels, often to supersaturation levels.