Structured interviews measuring tuberculosis knowledge were administered to 494 New York City injection drug users, 31% of whom reported a history of having a reactive tuberculin skin test. Medical records review of a subsample confirmed the validity of self-reported data. Most respondents understood the mechanisms of tuberculosis transmission. Three fourths of the subjects did not fully understand the distinction between a reactive skin test and active tuberculosis, but those who reported a history of skin test reactivity were twice as likely to understand this distinction. Forty percent of subjects did not understand the importance of medication adherence. Misunderstandings, based on a recent lack of tuberculosis education, may contribute to the fear and confusion that interfere with efforts to control tuberculosis.