The objective of the study was to determine the predisposing factors and incidence of toxicity among AIDS patients treated with a nevirapine (NVP)-based regimen in clinical practice. A retrospective cohort study of representative samples of AIDS patients treated with a NVP-based regimen was performed. A total of 206 adult HIV/AIDS cases with median age (IQR) 33 years (range, 29-38 years), 51% male, treated between January 2004-December 2005, were included. Most (92.2%) of the patients were naïve to antiretroviral drug. The incidence of NVP toxicity was 1.09/100 person-months. The median onset time was 4 weeks post NVP initiation (2.57 weeks for skin toxicity and 12.43 weeks for hepatic toxicity). History of drug allergy and NVP toxicity were significantly associated (p = 0.006), as were sulfamethoxazole allergy and toxicity (p = 0.015). Regarding concomitant medication, concurrent anti-tuberculosis drugs significantly increased the risk of NVP associated liver toxicity (p = 0.001). Therefore, it is important to monitor adverse events from NVP, including liver function tests among HIV/AIDS patients with history of drug allergy, especially against sulfamethoxazole, and those concurrently treated with antituberculosis drugs.