OBJECTIVE: We evaluated the relationship between 2 types of social relationships, ie, (1) external support for use of HIV pre-exposure prophylaxis (PrEP) and related study supplies and (2) participants' disclosure of PrEP use and condom use and HIV PrEP adherence among daily-dosing regimen participants in HIV Prevention Trials Network (HPTN) 067, an open-label trial of oral tenofovir (TFV) disoproxil fumarate (TDF) 300 mg/emtricitabine (FTC) 200 mg. METHODS: Using HPTN 067 survey data, we developed scales examining (1) Low Perceived External Support for PrEP: low perceived support by others for PrEP use or perceived negative reactions to the pill case (scoring ranges from 0 to 2) and (2) Participant-Staff Disclosure Challenges Scale, which identifies challenges to sharing nonuse of PrEP or condoms to study staff (scoring ranges from 0 to 4); these scales are the primary independent variables. Adherence, the dependent variable, was determined using log-transformed plasma TFV concentrations. generalized estimating equation (GEE) linear regression was used to assess the association between both scales and adherence. RESULTS: Participants (n = 161) included HIV-uninfected women in South Africa, and men who have sex with men and transgender women, in Thailand and the United States. In multivariable analyses, higher scores in the Participant-Staff Disclosure Challenges Scale were significantly associated with lower PrEP adherence [exp(beta) = 0.62, 95% CI: (0.46 to 0.84); P = 0.002] as were increased days since the last PrEP dose [exp(beta) = 0.73, 95% CI: (0.65 to 0.83); P