We report on a 60-year-old male with AIDS who presented Horner's syndrome that was associated with mononeuritis multiplex due to cytomegalovirus (CMV) infection. This is the first case who presented Homer's syndrome in the course of AIDS. Horner's syndrome associated with mononeuritis multiplex in this patient was the initial manifestation without any opportunistic infections. Since Horner's syndrome and mononeuritis multiplex in the present case were both improved by ganciclovir, it is important to consider CMV infection when Horner's syndrome or mononeuritis multiplex is observed in immunocompromised patients, such as those with HIV-1 infection or AIDS, even if they do not show any other opportunistic infections.