Nevirapine (NVP)-based antiretroviral therapy (ART) has been commonly used in many developing countries due to its affordability and feasibility. Nonetheless, the potential drug-drug interaction between NVP and fluconazole (FLU) is a major concern. NVP can induce cytochrome P450 isoenzymes in the liver while FLU inhibit the activity of this enzyme. The recent report has demonstrated that fluconazole significantly raises plasma NVP levels and may cause serious hepatotoxicity. Conversely, NVP does not significantly influence the plasma level of FLU. However, there have not been enough data or any recommendations to adjust NVP dosage for the concurrent use of both drugs in order to avoid the adverse events. A previous study has demonstrated that genetic disposition may play a role in NVP hypersensitivity reactions. There is little data of safety and tolerability for concurrent use of NVP and FLU in Asian populations. We therefore conducted this prospective observational study to compare the trough plasma NVP levels and frequencies of adverse events among antiretroviral HIV-infected patients who did not receive FLU and received FLU in different dosages for cryptococcosis prophylaxis or treatment; and subsequently received NVP-based ART regimens.