This cross-sectional study identified variables associated with protease inhibitor (PI) non-adherence in 179 patients taking anti-retroviral therapy. Univariate analyses identified 11 variables associated with PI non-adherence. Multiple logistic regression modelling identified three predictors of PI non-adherence: low adherence self-efficacy and seriousness of non-adherence and HIV (p less than 0.001), perceived absence of HIV associated illness (p less than 0.01), and use of more than one type of recreational drug (p = 0.001). The model correctly classified 83.9% of the sample, offers psychologists insight into psychological barriers to treatment adherence to guide interventions for improving adherence, and supports a modified version of the reformulated health belief model. (Contains 2 tables.)