Most adults are asymptomatically infected with Epstein-Barr virus (EBV). Primary EBV infection is commonly associated with acute infectious mononucleosis (IM). T cell immune activation plays an important role in EBV-associated diseases. IM shows a mainly Th1-type profile, so Th1-type cytokines such as interleukin-2 (IL-2), interferon-gamma (IFN-gamma), and lymphotoxin (LT) are moderately enhanced. We measured IL-2 and IFN-gamma in serum during acute phase of the disease and during convalescence. Sera were collected from 23 IM patients, 13 patients with similar clinical manifestations but without IM, and 10 healthy donors. The levels of IL-2, IFN-gamma and IL-12 were significantly higher in patients with acute IM than in healthy individuals. IL-2, IFN-gamma and IL-12 decreased during convalescence. These three cytokines may be useful as sensitive markers of IM during severe illness and its later phases.