Although adherence to clinic visits is important for successful clinical outcome in HIV treatment, little is known about the reasons patients with HIV miss appointments. We prospectively monitored the self-reported reasons for missed clinic appointments among HIV patients at a university hospital in South Korea from June through December 2006. Of the 1562 scheduled clinic visits, 131 (8%) of appointments were missed. Work schedule, forgetfulness and lack of motivation were the primary reasons given for missed appointments. Considering these factors when scheduling appointments may improve adherence to clinic attendance among HIV-infected patients.