Hepatitis C virus (HCV) infection is a major cause of liver disease and hepatocellular carcinoma in the United States. Of the estimated 2.7--3.9 million persons with active HCV infection, most were born during 1945--1964 and likely were infected during the 1970s and 1980s, before the advent of prevention measures. Nationwide, rates of acute, symptomatic HCV infection declined during 1992--2005 and then began to level. Declines also were observed in rates of newly reported HCV infection in Massachusetts. Although these declines were evident among reported cases overall in Massachusetts during 2002--2006, an increase was observed among cases in the 15--24 year age group. In response to this increase, the Massachusetts Department of Public Health (MDPH) launched a surveillance initiative to collect more detailed information on cases reported during 2007--2009 among this younger age group and to examine the data for trends through 2009. This report describes results of both efforts, which revealed continued increases in rates of newly reported HCV infection among persons aged 15--24 years. These cases were reported from all areas of the state, occurred predominantly among non-Hispanic white persons, and were equally distributed among males and females. Of cases with available risk data, injection drug use (IDU) was the most common risk factor for HCV transmission. The increase in case reports appears to represent an epidemic of HCV infection related to IDU among new populations of adolescents and young adults in Massachusetts. The findings indicate the need for enhanced surveillance of HCV infection and intensified hepatitis C prevention efforts targeting adolescents and young adults.