Cohort studies of young (aged 18-30 years) injecting drug users recruited in 1997-1999 in the Harlem and Lower East Side areas of New York City, New York, were used to assess the incidence of human immunodeficiency virus (HIV), hepatitis B virus (HBV), and hepatitis C virus (HCV). The authors found that HIV incidence was low at both sites: 0.8/100 person-years at the Harlem site and 0/100 person-years at the Lower East Side site. In contrast, HBV incidence was moderate (12.2/100 person-years) at the Harlem site and high (30.7/100 person-years) at the Lower East Side site. Similarly, HCV incidence was moderate (9.3/100 person-years) at the Harlem site and high (34.0/100 person-years) at the Lower East Side site. Results show that high rates of HBV and HCV transmission do not imply high rates of HIV transmission, even within an area of high HIV seroprevalence.