The concentration of total sialic acid (TSA) is increased in the plasma of patients with many types of cancer. The purpose of this study was to assess the usefulness of the TSA marker in predicting the efficacy of the treatment, and to compare TSA with two common markers, carcinoembryonic antigen (CEA) and the carbohydrate antigen 19-9 (CA 19-9). The study was performed on 44 patients treated for advanced colorectal carcinoma by a weekly 8 h continuous infusion of 5-fluorouracil (1300 mg/m2) plus bolus injection of L-folinic acid (100 mg/m2). TSA, CEA and CA 19-9 levels were measured before and after 3 months of treatment and their variations analysed as a function of the response to the treatment. TSA levels of patients with metastatic colorectal carcinoma before treatment (959 +/- 265 mg/l) were significantly higher than those of 32 healthy people (584 +/- 99 mg/l). The percentage of patients with TSA concentration above the cut-off level (782 mg/l) was 73% before treatment and 23% after. All patients who experienced an objective response to the treatment (complete, partial or minor response) (n = 29) had a significant decrease of TSA levels (t = 5.96; P < 0.001). When the disease was considered as stabilised (n = 10), TSA changed slightly, but it increased with progressive disease (4 out of 5 patients). Changes in CEA and CA 19-9 did not correlate as well as TSA to the treatment efficacy. Initial levels of TSA did not permit prediction of the efficacy of the treatment since they were not significantly different between the five response groups. TSA seems to be more likely involved in tumour changes than in tumour volume. Its determination could provide useful information about the spreading and metastatic properties of the tumour. TSA normalisation is an indicator of probable tumour growth arrest and its elevation could be a marker of relapse.