A patient with plaque stage mycosis fungoides (MF) developed an atypical disseminated cutaneous herpes simplex virus (HSV) infection manifested by polycyclic cutaneous ulcers. Although Tzanck preparations and serial antibody titers to herpes virus were negative, the diagnosis was readily established by viral culture and histologic examination of the skin lesions. Following adenine arabinoside therapy, the viral cultures of the ulcers became negative and the spread of virus-induced ulcerations ceased. In an immunocompromised host with rapidly advancing, sharply punched-out polycyclic cutaneous ulcerations, herpes simplex infection should be considered even though the classical vesicular lesions are absent.