Tumor necrosis factor-alpha (TNF-alpha) is important for function, differentiation, and transformation of B-lymphocytes in multiple myeloma (MM) but can also induce apoptosis of myeloma cells. Based on this opposite effect, it is very crucial to analyze the correlation of the serum level of TNF-alpha with clinical parameters of the patients. In this article, we analyzed 18 MM patients, 48% male and 52% female, with a mean age of 52 yr (range: 35-81 yr), clinical stage I in 21.4%, stage II in 26.4%, and stage III in 52.2% of patients. Patients with advanced clinical stage, presence of osteolysis, and elevated lactate dehydrogenase (LDH) had a significant difference (Mann-Whitney U-test, p < 0.05) in the serum level of TNF-alpha in comparison with those in the early stage, without osteolysis, and normal LDH. The correlation of individual values of TNF-alpha with the percentage of plasma cells in the bone marrow, LDH, beta2-microglobulin, fibrinogen, and sedimentation rate was significant (p < 0.05). However, we have not found a significant correlation between TNF-alpha and concentration of hemoglobin, the number of white blood cells or platelets (p > 0.05). We concluded that our data indicate determination of TNF-alpha as a good parameter for estimation of tumor mass presence, among individual patients with MM, and may by used for monitoring during application of different therapy protocols.