The diagnosis of infective endocarditis is based mainly on clinical features, echocardiography and blood culture findings. The study aim was to investigate serum cytokine levels in patients with infective endocarditis, and the possible use of these data in the diagnosis and monitoring of the disease. The study group comprised 40 patients with acquired rheumatic valvular heart disease and ongoing infective endocarditis. Two control groups had acquired rheumatic valvular heart disease: 15 patients without infective endocarditis, and 15 with urinary tract infection. Concentrations of serum interleukin-1a (IL-1a), interleukin-6 (IL-6) and a tumor necrosis factor-a (TNF-a) were measured three times during antimicrobial treatment. Serum IL-1a and TNF-a levels were not raised in the study group, or in controls; however, serum IL-6 levels were elevated in all patients with infective endocarditis, with a significant tendency to decrease during treatment. In controls without infective endocarditis, serum IL-6 levels were below calibration range, and in the group with urinary tract infection were slightly (but not significantly) elevated. In conclusion, elevated serum IL-6 levels may suggest ongoing infective endocarditis, and might serve as an additional parameter for diagnosis and monitoring of disease treatment. Serum IL-1a and TNF-a levels were not affected in infective endocarditis.