Creatinine clearance values were determined by duplicate urine collections in 18 patients with spinal cord injury, cerebrovascular accident, or multiple sclerosis. Measured creatinine clearance values were compared with estimates predicted by a urine-free mathematical method for estimation of renal function. Measured creatinine clearance values were considerably lower than would be ordinarily expected on the basis of patient body size, age, sex, and serum creatinine. In addition, creatinine clearances calculated using the urine-free method were considerably higher than measured values, suggesting that techniques for the prediction of creatinine clearance may not be routinely applicable in patients with these conditions.