Interleukin-8 (IL-8 or CXCL8) is a highly selective pro-inflammatory chemokine, that is elevated in sera of humans and animals with various inflammatory diseases. CXCL8 is possibly involved in uncontrolled inflammation and the development of a systemic inflammatory response syndrome (SIRS) and sepsis. Nevertheless, its behavior and precise properties in the course of inflammation are not fully understood. Thus, we used naturally occurring canine pyometra as a model of inflammation, in order to examine the behavior of serum CXCL8 in relation to the disease intensity and commonly analyzed inflammatory mediators. Using a commercially available canine ELISA kit, a significant increase of CXCL8 was determined in the serum of 23 dogs with pyometra compared with 35 healthy dogs. Interestingly, serum CXCL8 did not increase in severely diseased patients and behaved contrary to white blood cells (WBC), neutrophils and C-reactive protein (CRP). The measurement of serum CXCL8 may provide valuable information about the extent of ongoing lesions and could be a useful complement for existing laboratory tests.