To control hyperphosphataemia without hyperaluminaemia, A1(OH)3, which was given in addition to high doses of oral calcium, was replaced by Mg(OH)2 for 6 months in 20 haemodialysed patients and for 20 months in 12. The treatment during the control period was 110 +/- 91 mmol/day of oral calcium element given as CaCO3 and/or Calcium Sorbisterit and 1.05 +/- 1.47 g/day of A1(OH)3. Haemodialysis treatment was 4 h, thrice weekly. To prevent hypermagnesaemia, dialysate magnesium was decreased from 0.75 mmol/l to 0.375 mmol/l. After a control period of 3 months, Mg(OH)2 was given at a mean dose of 2.6 +/- 2 g/day and oral calcium supplements were decreased to 76 mmol/day. Two subsequent bone histomorphometry studies were performed at 8 month intervals in four patients and at 20 month intervals in seven patients. The results show a good control of plasma calcium (mean +/- SD: 2.43 +/- 0.1 mumol/l); phosphate (1.76 +/- 0.4 to 1.66 +/- 0.3 mmol/l); aluminum (1.3 +/- 0.1 mumol/l to 0.6 +/- 0.1 mumol/l); alkaline phosphatase (135 +/- 65 to 125 +/- 40 IU); and PTH fragments (PTH C terminal decreased from 260 +/- 214 to 185 +/- 182 pg/ml, PTH medium from 4185 +/- 5113 to 2270 +/- 4880 pg/ml). Plasma magnesium increased from 0.96 +/- 0.2 to 1.54 +/- 0.2 mmol/l. Bone histomorphometry shows no change in mineralisation, and a borderline decrease of resorption parameters. The main side-effects are (1) diarrhoea, which was well controlled by transient treatment with karaya gum, and (2) an increased need for potassium binders.(ABSTRACT TRUNCATED AT 250 WORDS)