BACKGROUND:HIV pre-exposure prophylaxis (PrEP) is a highly effective biomedical HIV prevention strategy, yet some gay and bisexual men (GBM) who are eligible to access PrEP are not using it. We report the incidence of PrEP uptake, factors predicting its initiation, and identify characteristics associated with nonuptake of PrEP among Australian GBM who meet the eligibility criteria. METHODS:The Following Lives Undergoing Change (Flux) Study is a national, online, prospective observational study among GBM focusing on licit and illicit drug use. Participants (N = 1257) responded to baseline and 6-monthly follow-up questionnaires. Incidence per 100 person-years and incidence rate ratios of PrEP initiation are presented. Multivariate Poisson regression was used to examine associations with PrEP initiation and logistic regression to examine associations with nonuptake of PrEP among eligible GBM. RESULTS:Among GBM who met the eligibility criteria, 69.8% of men did not commence PrEP. Factors independently associated with nonuptake of PrEP were younger age, living in an Australian state without a PrEP trial, lower social engagement with other gay men, less use of illicit party drugs or use of illicit party drugs for sex, and less likely to have engaged in HIV sexual risk behaviors such as group sex or any condomless anal intercourse. CONCLUSIONS:Despite meeting formal eligibility criteria for PrEP, men who were relatively less sexually active or less socially connected were less likely to initiate PrEP. Men who did not initiate PrEP may assess their risk as insufficient relative to others to warrant using PrEP because they engaged in less frequent "risky" behaviors.