This cross-sectional survey examined IDUs' understanding of HCV, their knowledge of virus transmission, clinical markers, symptoms and treatment. IDUs were recruited through and interviewed at a range of settings in inner-city, suburban and regional sites of New South Wales, Australia; 149 IDUs were recruited. Recent HCV testing was reported by 74% of participants, but a number of IDUs were confused about the results of the various tests, e.g. 19% of participants who stated they had HCV believed they could not infect others or were immune. Significant gaps in IDUs' knowledge of HCV were uncovered with respect to transmission risks (48% believed HCV could be contracted from re-using one's own, unshared needle), symptoms and clinical markers (42% believed HCV antibodies gave protection against HCV infection), and treatment. Findings warrant further development of, and research into, strategies to improve IDUs' understanding of HCV.