We retrospectively evaluated 73 immunocompetent adult patients assisted at our Infectious Diseases Clinic between March 1999 and March 2004 who presented fever and asthenia, mild to moderate increase of transaminases and serological findings compatible with recent cytomegalovirus infection. We excluded patients with a history of transfusions, drug abuse, immunodeficiencies, preexistent hepatic impairment or serological findings compatible with acute hepatitis A, B and C (HAV, HBV, HCV) and Epstein-Barr virus (EBV). The laboratory diagnosis of recent cytomegalovirus infection was made by especific IgM detection (ELISA) or a significant increase of specific IgG. The most frequent symptoms were fever (85%) and asthenia (83%), followed by cephalea (25%), splenomegaly (20%), adenomegalies (22%), pharyngitis (25%), myalgias (25%) and hepatomegaly (19%). All the patients showed moderate increase of transaminases and lymphomonocytosis (73/73). In average, ALT was increased by 6 fold and AST by 3.5 fold. The clinical characteristics that differentiate CMV infection from Epstein-Barr infection are the lesser frequency of adenomegalies and pharyngitis in the former. The differential diagnosis of CMV infection with hepatic involvement from acute hepatitis A and B, is based on the absence of jaundice, the lower elevation of transaminases, the intense lymphomonocytosis and the presence of specific IgM against CMV that are characteristic of CMV infection. In conclusion, in previously healthy young adults with fever, intense asthenia, lymphomonocytosis and moderate increase in transaminases levels, cytomegalovirus infection should be investigated.