This document shows estimates of new HIV infections in the United States from 1977 to 2006 broken down by transmission category. The majority of infections were among men who have sex with men, followed by injecting drug use, and a combination of men who have sex with men and injecting drug use. A smaller number of infections were attributed to heterosexual transmission. The estimates are based on an extended back calculation model that specified periods where the number of HIV infections was assumed to remain constant.