The OHTN is an open cohort comprised of individuals who have provided documentation of a positive human immunodeficiency virus (HIV)-antibody test or other laboratory evidence of HIV infection, and are patients at a participating clinic throughout Ontario, Canada. The study was initiated in 1994 as the HIV Ontario Observational Database and recruited its first participant in 1995. In that first phase, over 3,000 participants were enrolled and, as of December 2006, 1,656 were still active participants. As of May 2024, the study reports over 8,000 participants enrolled that are living with HIV. On average, participants were 41 years old at enrolment. The majority (60.3%) were MSM (men who have sex with men), although a sizeable minority (14.6%) were female and/or were infected through other modes of transmission. A large proportion (42%) were diagnosed in the 1990s and most enrolled several years after diagnosis. Participants are followed up by attending participating clinics.

Recruitment is ongoing