It is well known that Al absorption is markedly enhanced by citrate. The aim of the study was to document whether low-dose citrate ingestion (4 g/day) contained in a well-known effervescent calcium supplement was sufficient to increase Al absorption in 16 normal volunteers and 15 subjects with stable chronic renal failure under conditions of either Ca carbonate or Al hydroxide supplementation. Serum and urine Al levels were measured using flameless atomic absorption spectrophotometry as previously described. After Ca carbonate plus Ca citrate ingestion, there was no rise over baseline (Ca Carbonate alone) serum or urine Al levels in either group. Ca carbonate and Al hydroxide taken together produced a significant rise in serum and urine Al levels in both groups. Maintaing Al hydroxide but substituting Ca citrate for the carbonate (same dose of elemental Ca) produced a further significant increment in serum Al (0.47 ± 0.28-1.15 ± 0.8 μmol/l; p < 0.001) and in urine Al(1.37 ± 0.46-5.77 ± 5.21 μmol/l; p < 0.001) in the chronic renal failure group as well as in serum Al (0.42 ± 0.2-0.76 ± 0.48 μmol/l; p < 0.001) and urine Al (2.70 ± 1.24-8.24 ± 3.96 μmol/l; p < 0.001 in the normal volunteer group. Due to decreased urine excretion, the increment in serum Al in the CRF group was significantly greater than in the normal subjects (p < 0.02). Thus small quantities of citrate present in effervescing Ca supplements can significantly enhance intestinal Al absorption even in normal subjects. All citrate-containing preparations are totally contra-indicated in chronic renal failure patients ingesting Al-containing compounds. Patients themselves should be warned of the dangers.