The aim of the study was to establish the changes in the main lymphocyte populations and their subpopulations in peripheral blood of patients with acute pancreatitis. Seventy-one patients were studied in the course of disease. They were classified on the basis of severity of pancreatitis--mild and severe form. Percentage and absolute numbers of CD19+ (B lymphocytes), CD2+ (total T cells), CD3+ (immunocompetent T cells), CD3+CD4+ (helper T cells), CD3+CD8+ (suppressor/cytotoxic lymphocytes), CD4+CD29+ and CD4+CD45-RA+ (helper/inducer and suppressor/inducer subpopulations of helper lymphocytes). Immunophenotype analysis was performed using a flowcytometer Epics XL-MCL, Coulter, USA and monoclonal antibodies by the same company. In the early stage of disease the fall of total CD2+, immunocompetent CD3+ T lymphocytes and their subpopulations (more expressed in the severe form) is an indicator of determining the severity of pancreatitis. These results along with the elevated CD4+CD29+ subpopulation are a sign of both activation of these links in the local immune reaction and development of secondary infectious complications. The observed fall in CD4+CD45-RA+ suggests increased risk of development of autoimmune process. The recovery of most of the studied parameter after day 3 shows unimpaired host compensatory capabilities for both forms of disease.