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Many questionnaires on adherence to antiretroviral therapy are in use, but the validity of patients’ responses has not been tested. The Medication Adherence Self-Report Inventory (MASRI) has been developed and tested for its validity against objective measures and treatment outcome.Prospective study comparing questionnaire responses with MEMS TrackCap (MC, a medication event monitoring system), pill count (PC) and plasma HIV viraemia in a publicly funded specialist HIV clinic.Patients self-medicating antiretroviral therapy who were not cognitively impaired and were able to read and understand English.Mean adherence by MC of the 78 subjects was 92.9% (SE, 1.8%) and by PC 96.8% (SE, 1.4%). Agreement between MC and responses to items about doses missed 1, 2 or 3 days ago was low (κ = 0.23 (P < 0.03), 0.44 (P < 0.001) and 0.28 (P < 0.01) respectively). This improved when these responses were summated (κ = 0.46;P < 0.001) and was similar to that for recall of non-adherence over the preceding 2 weeks (κ = 0.54;P < 0.001). Mean self-reported adherence by visual analogue scale (VAS) over the preceding month was 93.3% (SE, 1.2%). This was strongly associated with both MC (r = 0.63;P < 0.001) and PC (r = 0.75;P < 0.001). On multivariate analysis, the strongest association between a MASRI item and MC was for the VAS. Both the 2 week recall and VAS items were inversely associated with viral load (P = 0.01). There was no association between dose timing (measured MC or questionnaire) or 3 day self-report and viral load.The MASRI provides a means of measuring patient adherence that is valid when compared with objective measures.