Data are sparse on injection drug using (IDU) men who have sex with men (MSM). Previous literature suggests perceived taboos can result in an underreporting of atypical sexual orientation (i.e., bisexuality, homosexuality). As a result, HIV prevention programs have been difficult to mount, particularly programs for IDU-MSM. The association between self-reported sexual orientation and sexual behavior at semi-annual study visits was longitudinally assessed in a population of 1300 male IDUs in Baltimore during the period 1993 to 1998. Overall, a small minority (5%) of the male IDUs inconsistently reported their sexual orientation over time. Logistic regression analyses were performed, which yielded five significant predictors. These men tended to be older, to have been incarcerated, to have attended shooting galleries during follow-up, and were more than twice as likely to be HIV-seropositive (OR, 2.66; 95% CI, 1.62-4.36) compared with those who consistently reported their sexual orientation. Furthermore, men reporting inconsistent sexual orientation tended to engage in higher risk behaviors, suggesting that these men should be especially targeted for interventions.