The serum levels of PTH, Mg, Ca, inorganic phosphate (Pi) and alkaline phosphatases (AlkPase) were determined in 22 uraemic patients on chronic haemodialysis with different Mg levels in th dialysate, in an attempt to clarify the pathogenesis of uraemic osteodystrophy. Baseline levels of all the considered parameters were obtained over a four month period whilst on standard Mg concentration in the dialysis solution (1.5 mEq/L). Patients were then divided into three groups: 10 patients were dialysed for six months with 0.5mEq/L of Mg, seven patients with 1.5mEq/L and five patients with 2.5mEq/L in the dialysate. At the end of the six months with differentiated Mg dialysis, the three groups had significantly different Mg serum levels, whereas no significant changes were observed in the PTH and Ca serum levels. All the patients on high-Mg dialysis showed a significant reduction of the Pi serum levels, whereas a significant increase of AlkPase was observed in the low-Mg dialysis group. The overall results obtained in the present study indicate a possible beneficial effect of low-Mg dialysis on the progression of uraemic osteodystrophy of patients on maintenance haemodialysis.