The purpose of our study was to investigate the characteristics of severe form of influenza A (H1N1) pdm 2009 with a fatal outcome, given the comorbidities. Materials and methods. Medical histories of 105 people who died in hospitals of St. Petersburg for the period of the epidemic of 2015/16 served as material for analysis. The lethality caused by the pandemic virus type A/ H1N1 / 2009 pdm was higher in males. Most of the patients had concomitant chronic diseases in the anamnesis. Obesity was observed in 44.8% (47/105) of patients, diabetes mellitus — 28.5% (30/105), isolated heart disease — 19.0% (20/ 105), combined pathology — 48.6% (51/105). In the first biochemical analysis of blood, creatine phosphokinase, lactate dehydrogenase were increased, total protein and prothrombin consumption index were reduced. The patient's death occurred after 5 days of illness in 88.6% cases, in 11.4% — up to 5 days of illness (inclusive). The analysis of fatal cases up to 5 days of a disease and death from complications (2—4 week) didn't find significant differences in the character and frequency of comorbidity. Specific antiviral therapy has been assigned to all patients, but 48 hours later. Bilateral subtotal viral and bacterial pneumonia was identified on the section, in the majority of cases, in 70.5% with hemorrhagic component. 30% patients had cerebral oedema, 41% patients had severe toxic parenchymatouse degeneration of miocardium, liver and kidneys. The pathology of the cardiovascular system, diabetes and obesity worsen the prognosis of the disease. Increased creatine phosphokinase, lactate dehydrogenase, and reduced total protein and prothrombin consumption index can be considered as markers of severe influenza. The ineffectiveness of antiviral therapy due to its late appointment, thus timely initiation of etiotropic treatment is very impotent.