This study evaluates the factors associated with initiation of services in the Boston HAPPENS Program, which is a collaborative network of care consisting of multiservice outreach agencies, community health centers and hospitals, for human immunodeficiency virus (HIV)-positive and hard to reach youth who are 12-24 years old. The program served 2116 youth who were 19.8 +/- 2.9 years old; 64% female; 45% youth of color; 16% gay/lesbian, bisexual, or undecided; and 10% homeless or runaway. At first contact with the program, 56% received outreach services; and 91% received a health intervention. Among those receiving a health intervention, 55% had HIV counseling and testing services, 49% medical care, 24% case management, and 9% mental health services. HIV-positive youth needed more contacts before a first medical visit than those who were HIV-negative or untested (p < 0.001). Different kinds of service sites reached different populations of at-risk youth. Logistic regression modeling showed that for young women, older age, lesbian-bisexual orientation, substance use, high-risk sexual behaviours, and receiving outreach services at first contact were independent predictors of initiation of services at outreach agencies; however, unprotected sex with males, and pregnancy were associated with a greater likelihood of care at hospitals or community health centers. For young men, older age, Asian/other ethnicity, and substance abuse were associated with care at outreach agencies; however, positive HIV status and unprotected sex with females were associated with care at hospitals or community health centers. Comprehensive networks of care offering a continuum of services and a variety of entry routes and types of care sites are needed to connect underserved youth to health care.