Sialic acid levels were determined in serum and in both normal and tumour-derived tissues from 30 patients with colorectal cancer. Total sialic acid (TSA), bound sialic acid (BSA),TSA normalised to total protein (TSA/TP) and BSA normalised to total protein (BSA/TP) were significantly higher (P < 0.0001) in sera from patients than in normal subjects. We found a trend of increasing serum sialic acid levels (TSA/TP and BSA/TP) as the malignancy became more severe (i.e. Dukes' stages A to C). Comparison of sialic acid levels between normal and tumour-derived colorectal tissues indicated no statistically significant differences in TSA, BSA or FSA (free sialic acid) levels between both tissues; however, TSA/TP and BSA/TP values were significantly decreased (P < 0.05) in the tumoral tissue. In this study, the possible relation between serum and tumour sialic acid levels in colorectal cancer patients was investigated. Our results showed that in these patients there was no correlation between serum BSA and tumour BSA levels.