Despite the fact that nucleoside analogues, such as aciclovir and ganciclovir, and DNA-polymerase inhibitors, such as foscarnet, have a proven antiviral effect on oropharyngeal-Epstein-Barr virus (EBV) replication, they have been unable to show any effect on the severity or duration of infectious mononucleosis (IM), a condition for which there is currently no established treatment. Clinical symptoms may be due to an EBV-induced polyclonal humoral, as well as cellular, immunoreactivity with limited pathology caused by viral replication itself. However, despite an extensive immune response, 90% of tested IM patients (n = 36) had a spontaneous outgrowth of in vivo EBV-infected B-lymphocytes at onset of disease, indicating lack of specific EBV-restricted cellular cytotoxicity at this time. Establishment of an EBV-specific T-lymphocyte response occurred 90-180 days after onset of disease (human leukocyte antigen-restricted cytotoxicity against EBV-infected B-cells). Thus, development of a specific cytotoxic response was a gradual and slow process. Assessment of cytokine pattern, at the single cell level, was performed by immunocytochemical technique and by enzyme-linked immunosorbent assay. This revealed an increased production of interleukin (IL)-2, interferon (IFN)-gamma, IL-6 and tumour necrosis factor (TNF) beta in all IM patients. Those with disseminated disease were characterized by lack of IFN-gamma production. This loss was selective since in vitro stimulation with superantigen, such as streptococcal pyrogenic exotoxin A, induced a normal response. These patients lacked signs of EBV-specific T-cell cytotoxicity in vitro. Treatment with intravenous or subcutaneous IFN-gamma, 1.5 MU every second day, in combination with intravenous immunoglobulin G (0.5 g/kg three times per week) and oral aciclovir, 800 mg 5 times daily, has shown promising results in some patients. Cytokine production in tonsil tissue in 4 patients with fulminant IM and respiratory tract obstruction showed a concomitant expression of IL-2, IFN-gamma, IL-6, TNF beta, transforming growth factor (TGF) beta 1-3, granulocyte colony stimulating factor, granulocyte macrophage colony stimulating factor, IL-4 IL-1alpha, IL-beta and TNF alpha. The number of IL-2, IFN-gamma, IL-6 and TNF beta producing cells was significantly higher compared to tonsil tissue obtained from children with tonsillar hypertrophy. Thus, IM is associated with extensive local cytokine production. It is suggested that this extensive cytokine production is closely involved in the pathology of IM and that patients with atypical IM have a dysregulation in the cytokine network. However, the mechanism by which EBV-infected B-lymphocytes triggers this cytokine cascade is still unknown. These findings show the need for evaluation of patients with immunodeficiency and EBV-induced lymphoproliferative disorders and perhaps the introduction of new immunoregulatory treatment strategies.