Patent citations are extensively used as a measure of patent-quality. However, counting citations does not account for the fact that citations come from patents of different qualities, and that citations are of variable qualities. We develop a citation index which takes into account the cumulative quality of the citing patents. We apply this index to the 2,139,314 utility patents granted in the u.s. between 1975 and 1999. We study the properties of this index by year and by technological category, and analyse the links between patents. Copyright © 2006 Akadémiai Kiadó, Budapest.