Erythrocyte and plasma magnesium concentrations were measured in 168 subjects. Eighty three of the subjects were sickle-cell anaemia patients (sicklers) with haemoglobin genotype HbSS, and in the steady state; 30 were subjects who had the sickle-cell trait (HbAS), and 55 were subjects with the normal haemoglobin genotype HbAA. All the subjects were aged between 10 and 16 years. The sicklers had significantly lower erythrocyte (P less than 0.01) concentrations when compared with the normal subjects or subjects with the sickle-cell trait. Erythrocyte and plasma magnesium concentrations in subject with the sickle-cell trait were not significantly different from the corresponding values in the normal subjects. A significant negative correlation (r = -0.2661, P less than 0.02) was observed between erythrocyte magnesium and plasma magnesium in the sicklers only. However, magnesium concentrations in erythrocyte and plasma were not related to age, sex and physical stature in the three study groups. These results suggest that sickle-cell anaemia patients may be magnesium deficient.