This study evaluated the influence of cognitive reserve on neuropsychological test performance in 198 patients infected with the hepatitis C virus. IQ scores, educational level, and occupational rating were combined to calculate a Cognitive Reserve Score (CRS) for each patient. Similar to studies of infection with the human immunodeficiency virus, there was a significantly increased risk of impairment in neuropsychological test performance in individuals with lower CRSs. It is important to account for CRS when assessing cognitive findings in large-scale clinical trials. (JINS, 2007, 13, 687–692.)Financial relationships of the authors with Hoffmann-La Roche, Inc., are as follows: K.L. Lindsay is a consultant and receives research support; A.S.F. Lok is a consultant and receives grant support; and R.J. Fontana is on the speaker's bureau. Authors with no financial relationships related to this project are as follows: L.A. Bieliauskas, C. Back-Madruga, E.C. Wright, and Z. Kronfol.