This paper investigates physicians' judgements about adherence to antiretroviral treatment (ART) among patients who have been HIV-infected through injecting drug use (IDU). Comparisons were made between data collected from physicians at enrollment (January 1996 to January 1998) of a prospective cohort study (MANIF 2000) and self-declarations of 196 HIV-infected injecting drug users (IDUs) who have been prescribed ART. The likelihood of being perceived as 'adherent' by physicians was higher for women, patients of 30 years of age or older, with biological markers indicative of a healthier status, and who were perceived as 'free of injecting behaviour' and not in drug maintenance treatment. Although the proportion of non-adherent patients was similar in physicians' assessment (26.0%) and patients' self-declarations (27.0%), a strong discordance occurred: 60.4% of patients self-reporting non-adherence to ART (80.0% for those receiving a protease inhibitor) were classified as adherent by their prescribing physicians. The study suggests that a priori judgements based on clinical experience but also on social stereotypes interfere with physicians' assessment, and that physicians' decisions to initiate complex treatment regimens may further induce optimistic biases and an underestimation of the problems faced by IDU patients to adequately adhere to them.