The objective of this study was to explore the usefulness of the fractional excretion of magnesium (FEMg2+) in Nigerian chronic renal failure patients (mild to moderate) in determining the severity of renal insufficiency. Plasma and twenty-four hour urine samples were analysed for magnesium and creatinine in thirty-five chronic renal failure patients and twenty-five healthy controls. The mean fractional excretion of magnesium (FEMg2+) was 15.3%(13.6) in the patients and 6.1%(2.3) in the controls and this correlated negatively with the creatinine clearance in both groups (r = -0.343, p < 0.05) in patients and (r = -0.665, p < 0.001) in controls. There was a positive correlation between plasma creatinine and fractional excretion of magnesium in the patients (r = 0.463, p < 0.05) and controls (0.455, p < 0.05). FEMg2+ may therefore be a more sensitive indicator of renal insufficiency and may also be a better indicator of the progression of renal failure.