This cross-sectional study assessed beliefs about highly active antiretroviral therapy (HAART) and their association with reported adherence to HAART among 109 HIV-positive patients receiving HAART while attending an ambulatory care clinic in Brighton, UK. Patients' beliefs about the necessity for and concerns about HAART and their adherence to it were assessed using validated questionnaires. There was considerable variation in beliefs and reported adherence. A quarter of participants reported low adherence to HAART and this was related to concerns about adverse effects as well as to the way in which each individual balanced concerns against perceptions of necessity. Patients were significantly more likely to report low adherence rates if their concerns were high relative to their perceptions of personal need for HAART. Patients with higher CD4 counts were significantly more likely to hold a view of HAART in which perceptions of necessity were higher relative to their concerns about adverse effects. CD4 was not related to reported adherence. A comparison of most recent viral load results between high and low adherence group identified differences in the predicted direction but these were not statistically significant. These preliminary findings suggest that patients' perceptions of HAART are important determinants of adherence with implications for future research and clinical practice.