The purpose of this study was to determine concentrations of IL-2, IL-10, TGF-β1 in serum and T regulatory cell (Treg) percentage in peripheral blood of dogs with perianal tumours. Investigations were conducted on 32 male dogs of mixed breed. The animals were divided into 4 experimental groups and control group. The groups were established depending on the tumour malignancy degree and the type of dominant hormones. All measurements of serum cytokine concentrations were conducted by the use of commercial diagnostic ELISA kits. Treg lymphocyte percentage was measured by flow cytometry. In both groups with benign tumours cytokine levels decreased during therapy, whilst in groups with malignant tumors, in spite of applying anti-tumour therapy, concentrations of cytokines in serum markedly increased. The mean percentage of Treg lymphocytes in dogs with benign tumours (group I and II) was significantly lower than the mean percentage of these cells in control group at all time points, but after applying of anti-hormonal therapy, the significant increase of Treg percentage was observed compared to baseline values. By contrast, in both groups with malignant tumours (group III and IV), the mean percentage of Treg lymphocytes was significantly higher at the beginning of the experiment comparing with the control group as well as both groups with benign tumours and this percentage increased during anti-tumour therapy. The results of this study suggest that monitoring changes in cytokine serum concentrations and Treg percentage in the bloodstream during anti-hormonal therapy may constitute a subsidiary marker in the monitoring of therapy effectiveness, in prognosis the outcome of a disease or in differentiating tumour degree of malignancy.