The passage of hepatitis C virus (HCV) across the dialysis membrane is a controversial issue. We performed a study applying extreme conditions of permeability to the dialysis membrane and avoiding the use of heparin and dialysis bath that might interfere with polymerase chain reaction (PCR) results. We obtained samples from the ultrafiltrate at the beginning of 18 hemodialysis sessions carried out in 6 HCV RNA-positive patients. HCV RNA was detected by PCR in 3 (16.7%) ultrafiltrate samples belonging to 1 of the patients. HCV genotype was the same as that found in positive ultrafiltrate samples and in the serum corresponding to this patient. The viral load of this patient was under the levels detectable by the assay employed. Therefore, contamination of the ultrafiltrate may constitute a potential risk for HCV transmission in hemodialysis units.