As part of a larger investigation examining genetic immunity to HIV, we undertook a cross-sectional investigation of 97 HIV-seronegative men who have sex with men (MSM). Our aim was to better understand the factors to which these men attributed their HIV serostatus and to relate these attributions to sexual risk taking. Three beliefs were related to sexual risk taking with HIV-negative/status unknown casual partners: (a) medication treatment advances, (b) the low probability related to HIV transmission, and (c) a healthy immune system, capable of resisting infection. A multivariate regression model suggested that use of recreational drugs, in combination with the belief that treatment advances reduce the risk of HIV seroconversion, in part, may explain the frequency with which individuals engage in unprotected anal receptive intercourse. Our findings suggest that MSM who intentionally engage in unprotected anal sex may be influenced by perceptions that medical advances have mitigated the threat of HIV and corroborate previous studies depicting an intimate relationship between illicit drug use and sexual risk taking.