Clinical, hematologic, biochemical and serologic data are recorded in seven patients aged 40 to 78 years with heterophil-antibody positive infectious mononucleosis (HA+IM). Clinical observations included fever of 22 to 30 days' duration (five of seven patients), sore throat (six of seven patients), myalgia (five of seven patients) and prominent lymph adenopathy (two of seven patients). Initial blood smears revealed significant numbers of atypical lymphocytes in only five of seven patients; however, or serial testing, in the remaining two patients Downey cells developed to a degree seen in most young adult patients with infectious mononucleosis. Comparison of liver function data from these and younger patients suggests that abnormalities tend to be more marked in those in the older than in those in the younger age range. Serologic tests confirmed primary Epstein-Barr virus (EBV) infections in all seven patients based on detection of IgM antibodies to EB viral capsid antigen in specimens obtained early, but not late, in the course of the infection, transitory antibody responses to the D (diffuse) component of the EMB-induced early antigen complex, and the initial absence and later development of antibodies to the EBV-associated nuclear antigen. Thus, the serologic data did not differ from those seen in younger patients. These results show that infectious mononucleosis should be included in the differential diagnosis of fever, sore throat and myalgia with or without significant cervical adenopathy in elderly persons.