P18 aims to understand why a new generation of young men who have sex with men place themselves at risk for human immunodeficiency virus (HIV) transmission. It seeks to understand why some men exhibit risky behaviours as they emerge into adulthood while others do not. Participants are biologically male residents of the New York City metropolitan area, United States of America, who had sex with a man in the six months before baseline screening and did not have HIV. Participants were recruited in two waves. The original cohort includes almost 600 men, who were aged 18 to 19 years during baseline assessment between 2009 and 2011. The second-wave cohort includes almost 400 men, who were aged 22 to 23 years at baseline assessment between 2014 and 2016. Participants are followed up every 6 months, and some have participated in sub-studies: P18 Neighborhood and P18 Viral. The P18 Neighborhood study included 250 participants whose spatial mobility was tracked for 2 weeks, in addition to their participation in the standard study protocol.