Both naturally occurring and semi-synthetic calanolide compounds are potent anti-human immunodeficiency virus (HIV) agents. In fresh human cells, they are highly effective inhibitors against low passage clinical virus strains, including those representative of the various HIV-1 clade strains (A through F), syncytium-inducing (SI) and non-syncytium-inducing (NSI) isolates, and T-tropic and monocyte-tropic isolates. These compounds also exhibit an enhanced antiviral activity against one of the most prevalent non-nucleoside reverse transcriptase inhibitor (NNRTI)-resistant viruses that is engendered by the Y181C amino acid change in reverse transcriptase (RT). Further enhancement of activity is observed with RTs that possess the Y181C change together with AZT-resistant mutations. Moreover, when challenged with viruses containing Y181C and K103N dual mutations, calanolide compounds remain active. These dual mutations are highly resistant to all approved NNRTIs (eg, delavirdine, nevirapine and efavirenz). In cell culture assays, calanolide compounds, especially (+)-calanolide A, select primarily resistant viruses possessing the T139I amino acid change. This mutation appears to be unique to calanolides since it remains susceptible to other NNRTIs. Synergistic effects are observed in both cultured cells and animal models when calanolides are used in combination with other anti-HIV agents. Enzymatic analyses indicate that calanolides inhibit HIV-1 RT through a mechanism that affects both the Km for normal substrate dTTP and the Vmax, resulting in a mixed-type inhibition, which is different from that of other known NNRTIs. Two possible binding modes/sites at the HIV-1 RT enzyme have been suggested for (+)-calanolide A. Taken together, the calanolide compounds represent a novel and distinct subgroup of the NNRTI family and inclusion of a calanolide in a combination therapy may be clinically beneficial. Of particular interest is the use of calanolide in the treatment of patients who have failed other NNRTI therapy and developed the Y181C mutation or the Y181C/K103N dual mutations. Currently, (+)-calanolide A, the most potent in the series of calanolide compounds, is undergoing clinical investigation for safety and efficacy in HIV-infected individuals.