Primary infection of human immunodeficiency virus type 1 (HIV-1) is occasionally associated with common cold-like symptoms, and rarely with a self-limited illness resembling infectious mononucleosis. We report a 32-year-old man who presented with infectious mononucleosis-like blood picture on admission. Five days after admission he developed hepatic encephalopathy, which was ameliorated by administration of bolus corticosteroid. Based on the results of serologic studies, we diagnosed that he had primary HIV-1 infection. To our knowledge, this is the first published report of hepatic encephalopathy as a clinical manifestation of primary HIV-1 infection.