Human T-cell leukemia virus type-1 (HTLV-1) is associated with a number of human diseases. Although the mechanism by which the virus causes diseases is still not known, studies indicate that viral replication is critical for the development of HTLV-1 associated myelopathy, and initial studies suggested that blocking replication with reverse transcriptase inhibitors had a therapeutic effect. Therefore, based on the success of HIV-1 protease inhibitors, the HTLV-1 protease is also a potential target for chemotherapy. Furthermore, mutated residues in HIV-1 protease that confer drug resistance are frequently seen in equivalent positions of other retroviral proteases, like HTLV-1 protease. Therefore, comparison of HTLV-1 and HIV-1 proteases is expected to aid the rational design of broad spectrum inhibitors effective against various retroviral proteases, including the mutant HIV-1 enzymes appearing in drug resistance. This review describes the characteristics of HTLV-1 protease, makes comparison with HIV-1 protease, and discusses the status of inhibitor development for the HTLV-1 protease.