Aim. To establish the clinical and laboratory features of severe forms of acute viral hepatitis В (AVHB). Materials and methods. 141 patients (88 males and 53 females) with different forms of the disease were examined. Among them there were 30, 33, and 78 patients with mild, moderate, and severe AVHB, respectively. In 23 patients, the course of hepatitis was complicated by acute hepatic failure; 6 of them developed hepatic coma that resulted in death. All the patients underwent comprehensive clinical and biochemical studies; some of them did a special laboratory study including immunological monitoring, determination of some parameters of lipid peroxidation (LPO) and the antioxidative system (AOS) in the blood cells, examination of blood rheological properties (by the deformability and viscosity of erythrocytic biomembranes), and determination of the level of endogenous toxemia by the content of medium molecular-weight peptides in the plasma, red blood cells, and urine. Results. Pathogenetic features of severe forms of AVHB have been identified. These include: 1) a hyperimmune response to the invasion of mixed cellular and humoral virus, followed by hepatic macrophageal hyperactivation; 2) high activity of LPO with the development of AOS failure; 3) deterioration of erythrocytic rheological properties: the decreased deformability and increased viscosity of their biomembranes; 4) the maximal level of endogenous toxemia. Conclusion. New agents to enhance the efficiency of treatment of patients with severe AVHB should be sought in the following directions: immunomodulating, antioxidative, membrane-stabilizing, and disintoxicant ones.