Statistical associations between substance use and seroconversion among gay and bisexual men abound. However, these associations often ignore men's own interpretations of their seroconversion. Using in-depth interviews with gay and bisexual men who reported using drugs or alcohol at the time of their seroconversion, we identify how these men explain the events that led to HIV transmission. Whereas a small minority of respondents reported substance use to explain their seroconversion, the majority reported three competing explanations. These participants claimed that they lacked sufficient knowledge about the behavioral risks that led to their seroconversion; that their decision to engage in unsafe sex was because of negative personal affect; and that they "trusted the wrong person." We link these findings to prevention and suggest that gay and bisexual men who use substances for recreational purposes will benefit from prevention efforts designed to address issues of gay and bisexual men rather than substance-using men.