The authors analyzed renal function in 25 patients with progressive MS. The mean glomerular filtration rate (GFR) was 92 mL/min/1.73 m(2), compared to the predicted GFR of 110 (p < 0.001). Nine of the 25 (36%) patients had abnormally low GFR (<90). The mean serum creatinine for patients with MS was lower than predicted normal values and poorly estimated GFR using standard equations. These data document impaired renal function in patients with progressive MS and have implications for treatment of these patients with potentially nephrotoxic drugs.