Twenty hemodialysis patients were prospectively evaluated to determine if concomitant citrate and aluminum administration enhances the absorption of aluminum, thereby increasing the possibility of toxicity. The four-phase study consisted of phase I, a washout phase; phase II, an aluminum treatment phase; phase III, a treatment phase combining aluminum and soluble calcium citrate; and phase IV, a treatment phase with the patient’s original prestudy phosphate binder. Results disclosed a progressive rise in serum aluminum levels (μg/L) from 47 ± 8 (phase I) to 62 ± 12 (phase II) to 74 ± 13 (phase III) and a drop to 58 ± 12 (phase IV). The difference in levels between phases I and III was significant. Additionally, and despite the fact that serum calcium concentrations did not change, serum phosphate and immunoreactive parathyroid hormone concentrations were significantly lower when aluminum and citrate were used together. This suggests that citrate enhances the absorption of aluminum and therefore increases the possibility of toxicity in the patient with end-stage renal disease.