The PARTNER study was an observational study focusing on the risk of sexual HIV transmission when an HIV positive person is on treatment. 

The second phase enrolled gay male couples in which one partner was HIV positive and the other was HIV negative. It only enrolled couples that had already chosen not to use condoms on a regular basis when they entered the study. 

To be eligible for PARTNER2, the HIV positive person needed to be on HIV treatment (ART) at the time of enrolment. Participants also completed questionnaires every six months describing how often they had sex. Clinical data with HIV viral load monitoring of the HIV positive partner and HIV testing of the negative partner was also carried out every 6-12 months. 

The PARTNER study was set up to address a question of great relevance in the daily life of people living with HIV and was conducted in collaboration with representatives of HIV organisations in an effort to make the study as close as possible to participants and to the larger community of people living with HIV. 

The second phase of PARTNER was funded by the National Institute of Health Research UK, ViiV Healthcare, Gilead Sciences, Augustinus Fonden, A.P. Møller Fonden and the Danish National Research Foundation [grant number DNRF126] and is sponsored by CHIP, Rigshospitalet. Additionally, funding for participation of the Swiss centres has been provided by the Swiss Office of Public Health. The study is coordinated cooperatively between CHIP, Rigshospitalet, University of Copenhagen, and University College London.