The aim of this study was to determine the serum concentrations of acute phase proteins (APPs), including C-reactive protein (CRP), serum amyloid A (SAA) and haptoglobin (Hp) in dogs with circumanal gland tumours. Investigations were conducted on 39 male dogs of mixed breed. The animals were divided into four groups depending on the degree of tumour malignancy and type of hormones that were dominant in the bloodstream. All measurements of serum APPs were conducted by the use of commercial diagnostic kits. In dogs with benign tumours, the concentrations of each APP decreased during therapy, while in dogs with malignant tumours, despite anti-tumour therapy, concentrations of APPs in serum increased markedly. The results of this study suggest that changes in serum concentrations of CRP, SAA and Hp during anti-hormone therapy may be a reliable marker for differentiating tumour grade and degree of malignancy. Including APPs in routine diagnostics may assist with formulating a prognosis for the outcome of the disease, before implementing anti-tumour therapy. Moreover, monitoring the effectiveness of therapy may be possible based on the determination of serum APP concentrations.