A curvilinear relationship of potential type was observed between serum concentrations of cystatine C and creatinine in 187 patients attended at our Nephrology Department. This relationship became linear when considering the reverse values, but the estimation error was much higher than what is clinically acceptable, which prevents the interconversion of values. In 78 critical patients, in whom an antibiotic therapeutic monitoring was being carried out, serum creatinine concentrations in the reference range were observed to be associated with pathologic concentrations of cystatine C. In 22 regularly hemodialysed patients, using Hemophan or Cuprophan membranes, increments of approximately 5% for cystatine C and beta 2-microglobuline in post-dialysis samples were observed. The chronic administration of inducer antiepileptic drugs, capable of modifying the concentration of different plasmatic proteins and reducing the serum concentration of creatinine, does not seem to limit the applicability of cystatine C as marker for glomerular filtration. The routine measurement of cystatine C seems to be warranted for estimating the glomerular filtration rate in patients in whom creatinine synthesis might be impaired.