We reviewed the thirty cases of cytomegalovirus infections with occurred in previously healthy patients, hospitalised for fever from 1981 to 1992. Pregnant women, transplant recipients, HIV infected persons and all immunocompromised subjects were excluded. We observed 34 cases (18 women, 16 men) whose mean age was 34 years (17 to 79). Fever appeared progressively (73%), persisted more than 15 days (87%) and was well tolerated. The main functional symptoms were headaches, myalgia (53%), profuse sweat (50%), abdominal pain, diarrhea, recent loss of weight, dry cough (51%). Splenomegaly was present in 24% of the cases. Chest X ray was always normal. Differential blood count was always inverse and an authentic mononucleosis syndrome was present in 91%: it appeared mainly 13 days after onset of symptoms. Hepatic abnormalities were nearly constant, especially cytolytic (97%) (transaminases three or four times upper the normal limit) but also cholestatic (62%). Thrombopenia has been noticed once (48,000/mm3). Serological diagnosis was confirmed with Elisa test (anti CMV Ig M: 30 cases) or complement fixation test (seroconversion: one, significant increase of the titers: two). CMV viremia, studied in seven patients, was positive in three. Spontaneous or treated (NSAI in 30%) outcome was nearly always favourable (97%). Two patients presented severe complications: meningo encephalitis and spleen rupture. CMV infection in previously healthy patients has to be suspected, without waiting for the mononucleosis syndrome, in view of a prolonged, well tolerated febrile illness, without pharyngitis, associated with hyperlymphocytosis and mild cytolysis. A careful follow-up is needed to detect the rare but severe complications.