OBJECTIVES: To compare the direct health costs of HIV-positive patients reporting sub-optimal intake of antiretroviral therapy (ART) with those of patients reporting full adherence. METHODS: 546 subjects from the Italian multicenter observational study ICoNA (Italian Cohort Naive Antiretrovirals) were followed between 1997 and 2004. Non-adherence to ART was assessed by a self-administered questionnaire. Medical costs incurred by the National Health Service were calculated retrospectively as from first ART and expressed in constant 1997 prices. RESULTS: Mean time on ART was 5.75 years (range 1.04-7.77); mean HIV-RNA and CD4 cells at baseline were 4.75 log10 copies/ml (range 1.3-6.6) and 307/μl (range 0-1,309). Non-adherence was reported by 197 (36%) patients, who showed a higher number of new AIDS-defining events (p=0.01), of detectable viremia episodes (p<0.001), and of ART changes (p=0.01). Overall medical costs and ART costs per year were on average €6,392 (range €555-€42,183) and €5,373 (range €537-€35,582), respectively, and did not significantly differ between the two groups. Annual inpatient costs were higher in the non-adherent group (€432; 95%CI €256-€608) than in the adherent group (€198; 95%CI €127-€269; p<0.005). On multivariate linear regression, higher HIV-RNA, lower nadir and baseline CD4, fewer ART changes, and interaction between low adherence and number of therapy switches were independently associated with higher log-transformed ART costs per year. Older age, HCV co-infection, sub-optimal ART adherence, lower CD4 nadir, and higher baseline CD4 were independently associated with higher annual inpatient costs, based on multivariate tobit analysis. CONCLUSIONS: Non-adherence is common among HIV-positive patients and is associated with virological failure, disease progression, more frequent hospitalizations and treatment changes. Total and ART costs do not seem to be significantly affected by non-adherence, probably because of switches to simpler and less expensive treatment options, whereas inpatient costs are significantly increased by sub-optimal drug intake.