The HYM Study investigates the individual, familial, interpersonal, and community contexts in which drug use, HIV risk-related behaviours, and health-promoting and seeking behaviours take place among young men who have sex with men (YMSM). Participants included young men who self-identified as one or more of the following: gay, bisexual, uncertain about their sexual orientation or reported having had sex with a man. All participants were living in Los Angeles County, California, USA. The study recruited two cohorts. The first cohort, consisting of approximately 530 men aged 18 to 24, was recruited between 2005 and 2006. These participants completed five waves of assessments conducted at 6-month intervals. The second cohort, consisting of around 450 African American/Black and Hispanic/Latino men aged 16 to 24, was recruited between 2016 and 2017. These participants completed eight waves of assessments at 6-month intervals. A renewal for the second cohort began in 2020, with plans for eight additional waves of assessments to be completed at six-month intervals until 2025.