The influence of the acute phase protein alpha 1-acid glycoprotein (AGP) on the concentration of total sialic acid (TSA) in serum was investigated by assessing their degree of correlation in 115 clinically healthy dogs, 29 dogs with malignant mammary tumours, 12 dogs with various other malignant tumours, 12 dogs with benign mammary tumours and 10 dogs with various other benign tumours. Serum from dogs with malignant mammary tumours and other malignant tumours had a statistically significant correlation between AGP and TSA concentrations (Spearman correlation coefficient (rS) = 0.52, P = 0.0005, n = 41). The correlation was also statistically significant in dogs with benign mammary tumours and other benign tumours (rS = 0.48, P = 0.02, n = 22). The Spearman correlation coefficient was 0.51 (P = 0.0001, n = 63) in all dogs with tumours. This was also the case if only those dogs with levels of AGP comparable to healthy dogs (< 750 mg/l) were included in the analysis (rS = 0.42, P = 0.01, n = 56). In clinically healthy dogs, the correlation was not statistically significant (rS = 0.17, P = 0.07, n = 115). None of the four groups of dogs with tumours had changed serum AGP concentrations compared to clinically healthy dogs (all t-tests gave P values above 0.05). The serum concentrations of AGP did not correlate with the clinical stage of dogs with mammary tumours. In conclusion, AGP and TSA concentrations in serum are positively correlated in dogs with tumours, partially explaining the increase in serum TSA in these dogs. Increased sialylation of the AGP molecule in dogs with tumours might contribute to the increased serum TSA levels.