Incarceration is associated with multiple and concurrent partnerships, which are determinants of sexually transmitted infections (STI), including HIV. The associations between incarceration and high-risk sex partnerships may exist, in part, because incarceration disrupts stable sex partnerships, some of which are protective against high-risk sex partnerships. When investigating STI/HIV risk among those with incarceration histories, it is important to consider the potential role of drug use as a factor contributing to sexual risk behavior. First, incarceration's influence on sexual risk taking may be further heightened by drug-related effects on sexual behavior. Second, drug users may have fewer economic and social resources to manage the disruption of incarceration than nonusers of drugs, leaving this group particularly vulnerable to the disruptive effects of incarceration on sexual risk behavior. Using the 2002 National Survey of Family Growth, we conducted multivariable analyses to estimate associations between incarceration in the past 12 months and engagement in multiple partnerships, concurrent partnerships, and unprotected sex in the past 12 months, stratified by status of illicit drug use (defined as use of cocaine, crack, or injection drugs in the past 12 months), among adult men in the US. Illicit drug users were much more likely than nonusers of illicit drugs to have had concurrent partnerships (16% and 6%), multiple partnerships (45% and 18%), and unprotected sex (32% and 19%). Analyses adjusting for age, race, educational attainment, poverty status, marital status, cohabitation status, and age at first sex indicated that incarceration was associated with concurrent partnerships among nonusers of illicit drugs (adjusted prevalence ratio (aPR) 1.55, 95% confidence interval (CI) 1.06-2.22) and illicit drug users (aPR 2.14, 95% CI 1.07-4.29). While incarceration was also associated with multiple partnerships and unprotected sex among nonusers of illicit drugs (multiple partnerships: aPR 1.66, 95% CI 1.43-1.93; unprotected sex: aPR 1.99, 95% CI 1.45-2.72), incarceration was not associated with these behaviors among illicit drug users (multiple partnerships: aPR 1.03, 95% CI 0.79-1.35; unprotected sex: aPR 0.73, 95% CI 0.41-1.31); among illicit drug users, multiple partnerships and unprotected sex were common irrespective of incarceration history. These findings support the need for correctional facility- and community-based STI/HIV prevention efforts including STI/HIV education, testing, and care for current and former prisoners with and without drug use histories. Men with both illicit drug use and incarceration histories may experience particular vulnerability to STI/HIV, as a result of having disproportionate levels of concurrent partnerships and high levels of unprotected sex. We hypothesize that incarceration works in tandem with drug use and other adverse social and economic factors to increase sexual risk behavior. To establish whether incarceration is causally associated with high-risk sex partnerships and acquisition of STI/HIV, a longitudinal study that accurately measures incarceration, STI/HIV, and illicit drug use should be conducted to disentangle the specific effects of each variable of interest on risk behavior and STI/HIV acquisition.