The EXPLORE study evaluated a behavioral intervention to prevent HIV seroconversion among men who have sex with men (MSM). The present ancillary study enrolled 345 EXPLORE participants at one study site (Boston) and assessed high-risk sexual behavior with other EXPLORE participants. It also assessed sexual intentions across other EXPLORE participants, HIV-negative individuals, and unknown HIV serostatus partners. Thirty-one percent reported having sex with another EXPLORE participant: 27% unprotected receptive oral sex with ejaculation (UO), 30% unprotected insertive anal sex (UIA), and 34% reported unprotected receptive anal sex (URA). Significant relationships between intentions to engage in UO, UIA, and URA, and type of partner emerged with intentions to engage in UO, UIA, and URA higher in HIV-negative partners, other EXPLORE participants, and unknown-HIV serostatus partners. Future HIV-prevention studies recruiting MSM at increased sexual risk of HIV infection should address participants potentially becoming sexual partners with each other.