Cytokines are part of pathogenesis in many diseases. Their measurement could be interesting for diagnostic purposes. One cytokine which participates in different inflammatory and neoplastic diseases is interleukin-6 (IL-6). The aim of this study was to investigate the IL-6 serum concentration in dogs with different liver diseases to show if there is any association between the cytokine serum level and the disease aetiology or the degree of the disease. IL-6 was measured in dogs with acute hepatitis, chronic hepatitis of different degrees and primary and secondary liver tumours. The data were compared with clinically healthy dogs and dogs with extra-hepatic diseases. For measurement, a commercial ELISA Kit (R&D Systems) was used. Compared with clinically healthy dogs and dogs with diabetes mellitus, all dogs with an intra- or extra-hepatic inflammatory or neoplastic disease have increased serum levels of IL-6. Dogs with acute hepatitis have significantly increased IL-6 serum concentrations compared with dogs with chronic hepatitis (P < 0.05). No significant difference between mild and moderate chronic hepatitis exists (P > 0.05). Dogs with secondary liver tumours have significantly increased IL-6 serum concentrations in comparison to dogs with primary liver tumours (P < 0.01), but both groups have comparable IL-6 serum concentration to dogs with extra-hepatic tumours. Measurement of IL-6 serum concentration may help differentiate between acute and chronic hepatitis and between primary and secondary liver tumours. Further information about the aetiology of the liver disease cannot be obtained by measuring IL-6 in the serum.