Background. Carcinoids are neuroendocrine tumors. They produce physiologicallyactive substances which in higher concentrations could cause carcinoid syndrome and carcinoid heart disease. The characteristic changes in carcinoid heart disease are carcinoid endocardial plaques. Pathogenesis of carcinoid heart disease is not completely understood. Aim. We tried to find distinctions in biochemical markers between two groups of patientsČ those withcarcinoid heart disease and those without it. Through these differences wewanted to get some insight in the pathogenesis of carcinoid heart disease and to check the diagnostic value of these biochemical markers. We also intended to calculate the incidence of carcinoid heart disease in patients with carcinoid syndrome in Slovenia. Hypotheses. 1. Levels of chromogranin A, S-hidroxyindoleacetic acid (5-HIAA) and transforming growth factor-(31 (TGF-(31) are significantly higher in the group with carcinoid heart disease compared with the group without it. 2. Concentrations of 5-HIAA before treatment with octreotide are significantly higher in the group with carcinoidheart disease compared with the group without it. 3. Treatment with octreotide does not prevent the appearance of carcinoid heart disease. Methods. This prospective study included all patients in Slovenia who were at the time diagnosed and treated with carcinoid syndrome. According to the echocardiographic findings we stratified the patients into two groupsČ those with carcinoid heart disease and those without it. Serum concentrations of chromogranin A were measured using radioimmunoassay, urine conentrations of 5-HIAA were measured using high pressure liquid chromatography and serum concentrations of TGF-beta1 were measured using enzyme linked immunosorbent assay. Nonparametric tests were used for statistical analysis. (Abstract truncated at 2000 characters).