To project the impact of population aging on total U.S. health care per capita costs from 2000 to 2050 and for the range of clinical areas defined by Major Practice Categories (MPCs). Secondary data: HealthPartners health plan administrative data; U.S. Census Bureau population projections 2000-2050; and MEPS 2001 health care annual per capita costs. We calculate MPC-specific age and gender per capita cost rates using cross-sectional data for 2002-2003 and project U.S. changes by MPC due to aging from 2000 to 2050. HealthPartners data were grouped using purchased software. We developed and validated a method to include pharmacy costs for the uncovered. While total U.S. per capita costs due to aging from 2000 to 2050 are projected to increase 18 percent (0.3 percent annually), the impact by MPC ranges from a 55 percent increase in kidney disorders to a 12 percent decrease in pregnancy and infertility care. Over 80 percent of the increase in total per capita cost will result from just seven of the 22 total MPCs. Understanding the differential impact of aging on costs at clinically specific levels is important for resource planning, to effectively address future medical needs of the aging U.S. population.