The most dramatic example of defining the pathogenicity of influenza virus A/H5N1 strains is the higher fatality rate of avian influenza epidemic (>50%) occured in Southeast Asia in 1997 comparing to the pandemic caused by influenza virus A/H1N1 in 1918 (5-10%) which was recorded as the most destructive pandemic in the world. When considering the fatal/total case numbers (208/340) reported by World Health Organization in respect of December 14th, 2007, the mortality rate has now reached to 61 percent. Recent studies have shown that the high fatality rate of avian influenza virus infections is a consequence of an overactive inflammatory response and the severity of infection is closely related with virus-induced cytokine dysregulation. The most important feature of A/H5N1 immunopathogenesis is the appearence of hypercytokinemia ("cytokine storm") which is characterized by the extreme (exaggerated) production and secretion of large numbers and excessive levels of pro-inflammatory cytokines. This phenomenon is blamed on the emergence of lethal clinical symptoms such as extensive pulmonary oedema, acute bronchopneumoniae, alveolar haemorrhage, reactive haemophagocytosis, and acute respiratory distress syndrome, associated with necrosis and tissue destruction. Numerous in vitro, in vivo and clinical studies have pointed out that A/H5N1 viruses are very strong inducers of various cytokines and chemokines [Tumor Necrosis Factor (TNF)-alpha, Interferon (IFN)-gamma, IFN-alpha/beta, Interleukin (IL)-6, IL-1, MIP-1 (Macrophage Inflammatory Protein), MIG (Monokine Induced by IFN-gamma), IP-10 (Interferon-gamma-Inducible Protein), MCP-1 (Monocyte Chemoattractant Protein), RANTES (Regulated on Activation Normal T-cell Expressed and Secreted), IL-8], in both humans and animals. The privileged cells of cytokine storm are macrophages and CD8+ T-lymphocytes, while the primary contributor cytokines are TNF-alpha, IL-6 and IFN-gamma. It has been detected that, mutations of some viral genes (NS1, PB2, HA and NA) are responsible for the cytokine storm, by increasing the viral replication rate, expending the tissue tropism, facilitating the systemic invasion and emerging of resistance against the host antiviral response. It has been shown that Glu92 and Ala149 mutations, and carboxyl-terminal ESEV/EPEV motif of NS1 protein have been implicated as determinants of virulence for A/H5N1 strains. In addition, Lys627 mutation in PB2 protein, polybasic aminoacid mutations in the cleavage region of hemagglutinin (HA) polyprotein, and glycosylation and sialylation mutations in HA and neuraminidase (NA) proteins were found to enhance the immune-mediated patology of highly virulent A/H5N1 strains. In this review article, the immunopathogenesis of influenza infection and the mechanisms of cytokine storm caused by influenza A/H5N1 viruses have been discussed under the light of recent literature.