Acute/early HIV infection is a period of high HIV transmission. Consequently, early detection of HIV infection and targeted HIV prevention could prevent a significant proportion of new transmissions. As part of an NIMH-funded multisite study, we used in-depth interviews to explore understandings of acute HIV infection (AHI) among 34 individuals diagnosed with acute/early HIV infection in six U.S. cities. We found a marked lack of awareness of AHI-related acute retroviral symptoms and a lack of clarity about AHI testing methods. Most participants knew little about the meaning and/or consequences of AHI, particularly that it is a period of elevated infectiousness. Over time and after the acute stage of infection, many participants acquired understanding of AHI from varied sources, including the Internet, HIV-infected friends, and health clinic employees. There is a need to promote targeted education about AHI to reduce the rapid spread of HIV associated with acute/early infection within communities at risk for HIV.