Forty-four patients, including 26 adults and 18 children under 15 years of age, were referred for evaluation of recurrent or persistent illnesses, with symptoms including pharyngitis, lymphadenopathy, fever, headaches, arthralgia, fatigue, depression, dyslogia, and myalgia. Thirty-nine patients were positive for Epstein-Barr virus antibody with antibody levels compatible with active infection for at least 1 year. Antiviral capsid antigen and anti-early antigen titers of patients were significantly greater (p less than 0.001) than age-group-matched controls. The frequency, number, duration, and patterns of symptoms, as well as patient sex, were compared by age in study patients seropositive and seronegative for Epstein-Barr virus. Illness patterns were not associated with changes in specific antibody titers or clinical findings. Lymphocyte phenotype and function analyses were done in 11 of the 39 patients positive for Epstein-Barr virus antibody; no consistent differences from normal were found. Only 1 of 32 patients had circulating interferon, in contrast to 7 of 7 patients with acute infectious mononucleosis. There were many adverse consequences of the illness. Epstein-Barr virus infection may not be self-limiting, and the virus may be associated with clinically recognizable illness other than infectious mononucleosis in children as well as in adults.