The objective was to investigate the relationship of disease severity, health beliefs and medication adherence among HIV/AIDS patients. A survey was administered to 72 patients in three different stages of HIV/AIDS (CDC clinical categories A, B, C). Multivariate analyses revealed that there were no significant differences in patients' perceptions of the severity of HIV/AIDS or perceptions of the benefits and barriers for the treatment across three disease stages. However, the most severely ill patients (in stage C) perceived a higher risk of complications if they did not take their medicine as prescribed compared to asymptomatic patients (stage A) (p = 0.01). Also, patients in stages B and C were more adherent to their medications compared to patients in stage A (p = 0.007). Finally, perceived susceptibility-inaction was positively related to medication adherence (p = 0.005) and difficulty in following doctors' instructions was negatively related to patients' medication adherence (p = 0.009). In conclusion, patients' illness experiences are associated with their beliefs about the chances of developing complications if they do not adhere to their medications. Patients who have experienced more complications perceive a stronger relationship between medication non-adherence and AIDS-related complications, and are also more adherent to their medication regimen when compared to patients with no prior complications.