Previous reports indicate that individuals with sickle cell anaemia may be magnesium deficient. The role of renal excretion of magnesium in the pathogenesis of this low magnesium status was investigated in 25 patients with sickle cell anaemia (sicklers) in the steady state and in 25 age- and sex-matched controls with haemoglobin genotype AA. The clearance, excretion fraction and excretion rates of magnesium were enhanced in the sicklers. These patients also had significantly lower erythrocyte and plasma magnesium concentrations than the controls. However expressed, urinary magnesium showed a significant positive correlation with plasma magnesium and a weak negative correlation with erythrocyte magnesium. Furthermore, a significant negative correlation was observed between erythrocyte and plasma magnesium in the sicklers. These results suggest that urinary disposal of magnesium in rates and amounts inappropriate to the body stores of the cation may be one of the mechanisms accounting for hypomagnesaemia and low erythrocyte magnesium content in patients with sickle cell anaemia.