Common sources of shiga toxin-producing Escherichia coli (STEC) O157 infection have been identified by investigating outbreaks and by case-control studies of sporadic infections. We conducted an analysis to attribute STEC O157 infections ascertained in 1996 and 1999 by the Foodborne Diseases Active Surveillance Network (FoodNet) to sources. Multivariable models from two case-control studies conducted in FoodNet and outbreak investigations that occurred during the study years were used to calculate the annual number of infections attributable to six sources. Using the results of the outbreak investigations alone, 27% and 15% of infections were attributed to a source in 1996 and 1999, respectively. Combining information from both data sources, 65% of infections in 1996 and 34% of infections in 1999 were attributed. The results suggest that methods to incorporate data from multiple surveillance systems and over several years are needed to improve estimation of the number of illnesses attributable to exposure sources.