Persons with acute HIV infection (AHI) may be responsible for a substantial proportion of onward transmission of HIV infection. AHI is characterized by unfettered replication of HIV in a "ramp up viremia". The high concentration of HIV in blood and genital secretions remains elevated for up to 10-12 weeks before it declines to the levels observed in established infection. These high levels of HIV shedding in the genital tract are likely to produce very efficient sexual transmission and the proportion of virions that are infectious may be substantially higher during acute compared to chronic infection. Consequently, the probability of transmission during unprotected intercourse for those with AHI is very high. Identifying persons with AHI and intervening to reduce onward transmission represents a tantalizing, but unproven, opportunity for HIV prevention.