Cachexia is an unintended loss of >10% of body weight over 12 months and occurs in 50–80% of patients with cancer. It is associated with reduced quality of life, poor response to therapy, and reduced survival. Interleukin 6 (IL-6) plays an important role in cachexia pathogenesis, as it is an anti-inflammatory cytokine which regulates muscle mass maintenance. The purpose of this study was to examine the possibility of using salivary IL-6 in cachexia diagnostics. The study included 37 patients with the metastatic malignant disease. Unsimulated saliva samples were collected by passive spit method. Serum samples were collected by the venipuncture method of the cubital vein. Concentrations of salivary and serum IL-6 were determined by the enzyme-linked immunosorbent assay. Statistically significant differences were not found in salivary and serum IL-6 concentrations between patients with cachexia and patients without cachexia. The correlation was not found between salivary and serum IL-6 values in the observed groups. Taking into account all the limitations, the results of this study indicate that neither salivary nor serum IL-6 is a sufficiently sensitive marker for cachexia diagnosis in oncology patients. Further research with a considerably larger number of participants is necessary.