Disorders in immune reactions represent one of the main pathogenetic mechanisms in patients of Intensive Care Units who suffer from sepsis or syndrome of systemic inflammatory response. The determination of early activation sign CD69 on T lymphocytes makes it possible to evaluate functional capacity of this cell population. The determination of other selected immunological parameters contributes to differential diagnosis of infectious and non-infectious etiology of systemic inflammatory response. The authors demonstrated lower expression of CD69 on CD3+ CD8+ lymphocytes after stimulation with phytohemagglutinin in patients with sepsis as well as patients with the syndrome of systemic inflammatory response of non-infections etiology. No statistically significant differences were proved in the concentrations of IgG, IgA and IgM, number of leukocytes, percentual representation of lymphocytes, TNF alpha production and the expression of surface sign of CD64 in both groups of patients. There were statistically significant differences in the concentrations of C3 and C4 components of complement, prealbumin and procalcitonin. The study did not show correlation between the expression of CD69 and selected immunological parameters (IgA, IgM, IgG, C3, C4, leukocytes, lymphocytes, prealbumin, procalcitonin, TNF alpha and CD64).