We investigated the safety and efficacy of magnesium hydroxide as a phosphate binder in patients with end-stage renal disease on maintenance hemodialysis, 9 volunteers participated in a four-phase study during which each ingested (1) no phosphate binders, (2) magnesium hydroxide (Mg(OH)2) alone, (3) Mg(OH)2 and aluminum hydroxide (A1(OH)3) together and (4) A1(OH)3 alone. Serum magnesium (SMg) concentrations were maintained at less than 4.5 mEq/1 (2.3 mmol/l) in all subjects while they were ingesting 0.75-3 g Mg(OH)2/day and no magnesium toxicity was noted. In individuals taking a constant daily dose, SMg remained stable over 8-12 weeks. Serum phosphorus (SP) decreased from 9.0 mg/dl (2.9 mmol/l)d during the control period to 8.1 mg/dl (2.6 mmol/l) during the Mg(OH)2 period (p less than 0.05) and increased from 6.1 mg/dl (2.0 mmol/l) during the Mg(OH)2 and A1(OH)3 period to 7.0 mg/dl (2.3 mmol/l) during the Al(OH)3 period (p less than 0.05) indicating that Mg(OH)2 could significantly lower SP. However, SP was best controlled (6.1 mg/dl; 2.0 mmol/l) when Al(OH)3 and Mg(OH)2 were used together and all participants preferred the combination therapy to either of the agents alone. These results indicate that Mg(OH)2 is a potentially useful adjunct to A1(OH)3 for managing hyperphosphatemia in patients on maintenance hemodialysis. In this short-term study Mg(OH)3 for managing hyperphosphatemia in patients on maintenance hemodialysis. In this short-term study Mg(OH)2 was well tolerated and with appropriate monitoring did not cause uncontrolled hypermagnesemia. Further studies are clearly required to determine whether long-term therapy with Mg-containing agents is safe in dialysis patients.