Background Pandemics represent one of the few, true, global public health emergencies. Even if the timing of the next pandemic is uncertain, there is general acceptance that one is expected in the foreseeable future. The effect of demographic trends on the likely outcomes seen during a pandemic have not been studied in a quantitative form useful for planning and policy agencies. We estimated the excess mortality, morbidity, and health service utilisation due to pandemic influenza in the resident population of the United States that would be likely to occur if a pandemic was to take place in the present and project these estimates to the year 2050. Methods Simulation methods were used to estimate excess mortality, hospitalizations, outpatient visits, and illnesses (cases requiring at least half a day off work) for a pandemic with an attack rate (clinical cases per unit population) of 15% to 50% in increments of 5%. The current status of the US population and its projected demographic changes were available through the Bureau of Census. Age- and risk-specific rates for each of the four outcomes of interest were derived from a Monte Carlo simulation and summarized. Results Under the most conservative estimates (attack rate of 15%, lower estimate of high-risk population, P2.5 of simulated values), about 70,000 excess deaths would be expected if a pandemic were to take place in 2003. In 2050, excess deaths could number as high as 750,000 people (P2.5-P97.5 [approx] 470,000 to 1,050,000). About 800,000 (P2.5-P97.5 [approx] 380,000 to 1,300,000) excess hospitalizations would be expected in the event of a pandemic in 2003 at an attack rate of 30%. In 2050, a pandemic producing clinical symptoms in 50% of the population could result in 1,310,000 (P2.5-P97.5 [approx] 700,000 to 1,930,000) excess hospitalizations. A pandemic affecting 15% of the US resident population in 2003 will most likely result in 18,700,000 (P2.5-P97.5 [approx] 17,800,000 to 19,700,000) excess outpatient visits and about 23,000,000 (P2.5-P97.5 [approx] 21,700,000 to 24,200,000) illnesses. Conclusion Projected population changes are pivotal in considering general and local health planning and preventive medicine activities in the event of an influenza pandemic.