The Ontario HIV Treatment Network Cohort Study (OCS) is an observational, open dynamic cohort of people who are receiving medical care for human immunodeficiency virus (HIV) infection in Ontario, Canada. Established in the mid-1990s, the OCS has its roots in AIDS activists' demands for research that would improve the quality of life of people living with HIV while respecting their privacy. It is a collaborative and community-driven study, including a Governance Committee made up of people with HIV and other stakeholders that evaluates analysis project proposals for community relevance and ethics. From 1995 to 2010, a total of 5644 participants were enrolled and 27,720 person-years of observation were accumulated; follow-up will continue until at least 2015. In the initial years of study, the focus was on clinical data from medical chart reviews. It has since evolved into a comprehensive study that collects extensive de-identified information on clinical, laboratory and psychosocial and behavioural measures based on medical chart abstractions, interviews using a standardized questionnaire and linkage with external administrative health databases in Ontario. Interested collaborators are encouraged to submit analysis project proposals as instructed on the study website (www.ohtncohortstudy.ca).