Young men who have sex with men (YMSM) face a disproportionately high burden of HIV. Oral pre-exposure prophylaxis (PrEP) is effective in preventing HIV acquisition, but adherence to PrEP among YMSM may be inadequate. Medication adherence may be assessed via biomarkers, which are expensive and invasive, or via self-report through Audio Computer Assisted Self-Interview (ACASI), which may result in over-reporting of adherence. In this paper we assess the potential of a new method of self-report, the Interactive Questionnaire System (iQS), in validly estimating true adherence rates.