We estimated influenza- and respiratory syncytial virus (RSV)-associated hospitalizations by age, high-risk status and outcome, during the 1996/1997-1999/2000 respiratory seasons among adults who did not receive influenza vaccine. Using three health maintenance organization (HMO) databases and local viral surveillance data, we identified weeks when influenza and RSV were circulating and estimated influenza- and RSV-associated hospitalizations. Persons aged > or = 65 years with and without high-risk conditions had significantly increased rates of influenza-associated hospitalizations for pneumonia and influenza, and circulatory and respiratory diseases. Persons aged > or = 65 years with high-risk conditions also had significantly increased rates of influenza-associated hospitalizations for cardiac conditions (16.9 per 10,000 person periods). Relative to the influenza estimates for high-risk persons > or = 65 years, we found lower rates of RSV-associated hospitalizations for pneumonia and influenza diseases (23.4 per 10,000 person periods), cardiac diseases (4.3 per 10,000 person periods) and circulatory and respiratory diseases (44.0 per 10,000 person periods). Among low-risk persons aged 50-64 years, we did not identify significantly elevated rates of influenza- or RSV-associated hospitalizations. Excess hospitalization estimates among adults aged > or = 65 years and high-risk 50-64 year olds during the influenza season suggest that these groups should have priority for influenza vaccine during vaccine shortages.