Private sex parties are an emerging risk environment for HIV among men who have sex with men (MSM). In 2009, 103 participants who reported attending at least one sex party in Massachusetts in the prior 12 months completed an in-depth, interviewer-administered quantitative assessment. Multivariable logistic regression analyses were conducted to examine associations with having engaged in one or more serodiscordant unprotected anal sex (SDUAS) acts at the most recent sex party attended. Nearly one-third (32%) of the sample reported engaging in SDUAS at the most recent sex party attended. Adjusting for age, race/ethnicity, and educational attainment, variables associated with an increased odds of engaging in SDUAS at the most recent sex party were: total number of unprotected anal receptive sex acts at sex parties in the past 12 months, self-perception of being at-risk for transmitting or acquiring HIV, and sexual sensation seeking. Examined in the same model, if condoms were provided/available at the most recent sex party attended, participants were at a decreased odds of engaging in SDUAS at that sex party. The majority (80%) expressed an interest in HIV prevention activities for MSM who attend sex parties. HIV prevention interventions are needed to reach MSM who attend sex parties and should take into account individual and contextual factors that may contribute to sexual risk. Environmental factors in the sex party setting, in particular the presence and availability of condoms, may potentially mitigate individual-level factors such as unprotected anal sex.