Creatinine as a marker of renal function has limited value in Duchenne muscular dystrophy (DMD) because of reduced muscle mass. Alternative methods of assessing renal function are sorely needed. Cystatin C, a nonglycosylated protein unaffected by muscle mass, is potentially an ideal biomarker of nephrotoxicity for this population but requires validation. In all, 75 subjects were recruited: 35 DMD (mean age 10.8 +/- 5.4 years, corticosteroids n = 19, ambulatory n = 26), 29 healthy controls, 10 with renal disease, and one DMD with renal failure. Cystatin C levels in DMD were normal irrespective of age, ambulation, or corticosteroid treatment. Serum cystatin C was 0.67 +/- 0.11 mg/l compared to normal controls 0.69 +/- 0.09. mg/l. In these same individuals serum creatinine was severely reduced (0.27 +/- 0.12 mg/dl) versus normals (0.75 +/- 0.15 mg/dl, P < 0.01). In one DMD subject in renal failure, cystatin C was elevated. This study demonstrates the potential value of cystatin C as a biomarker for monitoring renal function in DMD. Its applicability extends to other neuromuscular diseases.