In this study we used grounded theory methodology to understand the influence of loneliness in YMSM in terms of HIV risk behavior. Twenty-two YMSM, 18-29 years of age, of HIV-negative/unknown status were interviewed. Results indicated that loneliness is defined in two ways: emotional and social. A cyclical pattern emerged that included negative symptoms, "self-treatment" of loneliness through drug use and sex, temporary relief, remorse related to engaging in HIV risk behaviors, negative self-image, and ultimately the reemergence of initial loneliness symptoms. Results suggest that loneliness in YMSM plays a greater role in HIV-related behavior than previously understood. A preliminary explanatory model, implications, and directions for future research are discussed.KEYWORDS young men who have sex with men (YMSM), HIV/AIDS, risk behaviors, qualitative research, grounded theory